This page in a nutshell: Substitution copies the content of a template and inserts it into a Wikipedia page once.
Substitution is the one-time, permanent copying of content onto a Wikipedia page. It differs from transclusion, which continually updates as the parent template changes. Substitution is most often used to handle templates, and this page lists templates that should either always or never be substituted.
Definition
Ordinarily, templates will be transcluded, which means that their content is always retrieved directly from the most recent version of the template. This is advantageous because, if the template's page is changed, every other page that uses that template will display the updated content. In this case, when someone is editing a page that transcludes a template, they only see the {{Template call}} and not the underlying code. This procedure is used for most templates to avoid having to manually keep pages up-to-date individually.
However, some templates can also be substituted. Substitution is basically a fancy "copy and paste" when it's used. In practice, once an editor substitutes a template on a page and publishes their edit, the entire content of the template at the moment it is substituted will be inserted into the page, along with any parameters passed to it, as if the editor had manually copied the code and pasted it. Since the substituted content is now an integral part of the page, it can be modified without affecting other pages or the parent template itself. This means that, if the template is changed in the future, the pages in which it has already been substituted will not be affected in any way, because there is no longer a connection between them and the substituted template.
Usage
To transclude a template, its name is simply enclosed in double curly braces, as in: {{Please ping}}. To substitute a template instead, the string subst: is added after the opening braces, as in {{subst:Please ping}}. The contents of {{Please ping}} will then be inserted ("expanded") in the location where the template call is placed in the page. Before saving, you can check the resulting wikitext, which differs from what you see in the editor, by clicking "Show changes".
Benefits
If there is a template you would like to modify for a single occasion, but you do not want to modify the template for all the pages that use it and you do not want to make a new template, you can substitute the template and then make the modifications with a second edit. For example, a template might be used with all the states of the United States and with the District of Columbia. The template might be worded correctly for the states, but not for DC. To fix the wording, you can substitute the template and then fix the wording.
Substituting frequently used but unprotected templates limits the damage that can be caused by people vandalizing the template. See high-risk templates.
Transcluding certain templates hides wikitext from newcomers, whereas substituting makes the wikitext visible and gives them the opportunity to learn its use (of course, there is a "how to edit" link at the bottom of each edit page).
Substitution allows for recursive evaluation through macro templates.
Substitution makes the resulting page independent of future changes to the template.
Substituting enables other automatic replacements to occur. For example, a template may contain four tildes, which will be replaced by the user's signature and timestamp only when the template is substituted.
Substituting enables a page's wikitext to be copied to another project without re-creating the templates contained in the page.
Neutral
Substituting en masse may ultimately speed up the site's page rendering speed, but this is not a reason to prefer substitution over transclusion. Don't worry about performance of Wikipedia's servers.
Templates are often modified or deleted. If a template is boilerplate text, consider whether you want it to vary as the template is modified. If your answer is "no", substitution is warranted. An example of this is the {{subst:welcome}} template. The archives of a user's talk page should show the actual welcome message they received, not the current welcome message.
Substituting makes the correspondence between wikitext and rendered page easier to understand, especially to new users. However, the opposite may also apply.
Drawbacks
Once a template is substituted, the result is no longer linked to the template, making it hard to find all pages displaying that text (though categories can sometimes relieve this). The problem can be worked around by including a link to the template in the template's code.
A substituted template will not be updated when errors in the master template are corrected.
If the template is used to standardize the appearance of something, you probably do not want to do a substitution. An example of this is a table of contents or navigation box.
Substituting en masse—editing thousands of articles with bots—temporarily slows down the site and wastes server resources.
Substitution increases the size of pages in the database and database dumps.
A substituted template can add a lot of wiki-code or HTML to the article, harming accessibility for the less technically inclined.
Substituting templates prevents newcomers from learning to use templates and prevents users from finding template documentation.
Substituted templates are much harder to remove or modify, especially if the page has been significantly modified around them.
When a vandalised template is substituted, it is more difficult to repair than regular vandalism because of the lack of links between the template and its incarnations and the lack of updatability.
Other users cannot see that the text was produced using a template (and will not learn to use the template). To mitigate this issue, many templates add a hidden comment such as <!-- Template:Please ping --> to the start or end of their output.
Some magic words and meta-templates, such as {{!}} and {{((}}, do not work if they are substituted.
Documenting substitution
Usage of a template through subst: does not automatically show up in page histories. Therefore, it is strongly suggested that the template be mentioned in the edit summary so other editors can easily understand your edits. (For instance, include "Substituted [[Template:Please ping]]" or "{{subst:Please ping}}".) If you don't, they might think you wrote the entire template code yourself.
HTML comments outside noinclude tags are included in the substituted wikitext, so a comment can be used to mention the template, and then find it again later. See any of the user warning templates, which all use this approach. Template {{uw-coi}}, for example, places the comment onto the page where it is substituted, thus enabling searching for that comment as a proxy for searching for substitutions of the template. The §Notes section of the template's documentation provides a search link based on this technique.
If you are creating or editing templates that are commonly substituted, you can add a hidden comment to the template so that other editors know which templates were substituted (e.g. <!-- Template:Please ping -->). Use {{subst:HTML comment}} if you want to write HTML comments that contain wikitext expansions.
Substitution of parameters does not work inside comments, but a workaround for this is to begin an intended comment with <!<noinclude/>-- so that it turns into a comment once transcluded.
Template-specific usage
This page also lists templates that should either always or never be substituted except in the Wikipedia namespace. Bots that do such replacements will never be used on the original template. If you are going to use a bot to substitute templates, please read through the talk page first, as many are under dispute or change status over time, and substitution is permanent.
These templates have a purpose and/or syntax that requires them to be substituted.
{{afd}}, {{afd2}}, {{afd3}}, {{rfd}}, {{tfd}}, {{tfdnotice}}, or else the link to "this article/redirect/template's entry" does not work. In addition, templates which document a finished deletion process ({{afd top}}, etc.) should be substituted.
{{cfd}}, {{cfr}}, {{cfm}}, {{cfr-speedy}}, which insert comments which serve as queues for JJMC89 bot III, which is used to rename or merge categories.
{{copyvio}}, for articles to be listed at copyright problems, will result in an error message if transcluded.
{{nld}}, for images with no info on copyright status, adds {{no license}} with today's date automatically filled in.
{{nsd}}, for images with no source info, adds {{di-no source}} with today's date automatically filled in.
{{orfud}}, for "generic" orphaned fair use images, adds {{di-orphaned fair use}} with today's date automatically filled in.
{{or-fu-re}}, for fair use images that have been orphaned in favour of another image, adds {{di-orphaned fair use}} with today's date automatically filled in (takes the name of the replacement image as parameter).
{{requested move}}, for requesting page moves, adds a new talk page header with today's date automatically filled in, among others.
{{rfu}}, for fair use images that it should be possible to replace with a free licensed image, adds {{replaceable fair use}}, with today's date automatically filled in.
{{refu-c}}, used in captions of images flagged as replaceable fair use, adds {{rfu-c}} with the "due date" (today + 7 days) automatically calculated and filled in.
{{OnProd}}, which inserts today's date-dependent name of the category page to which the article was added when nominated for deletion.
Templates that should always be substituted
This is a list of templates that should always be substituted. Each list is ordered alphabetically with grouped series. This is not a comprehensive list; other less-used templates may also need to be substituted. See the template's documentation for details.
{{Format ISBN}}, substituting these formats 9-digit SBN's, 10- and 13-digit ISBNs according to the rules determined by the International ISBN Agency.
{{No more links}}, substituting this inserts a commentary only visible in edit mode, with a short explanation to discourage spamming links.
User signature templates, if transcluded, cause undue server strain as changes to a minor user template must then be widely propagated. If used, user signature templates should be substituted. (It should be noted, however, that user signature templates are discouraged.)
leave some parameters to their defaults by not specifying them, unless the alternative default mechanism is used
contain external links, as the formatting of the URL to perform queries or look-ups may change
This is because these constructs are not replaced in the generated wiki-code (single level substituting of case (A) leaves the #if or #switch constructs verbatim at the subst location and (B) leaves constructs like {{{1|default value}}}).
If specific consensus is to eliminate a particular call of such a template, Special:ExpandTemplates can be used to expand that call to plain wiki-syntax.
List
This is a list of templates that should not be substituted. This is because they contain formatting standard code, complex code, or code that breaks if substituted.
Various citation/reference templates which contain very complex conditional code (see: WP:CITET).
Infobox templates, various ones for the sake of bot-identification and consistency.
{{Javadoc:EE}}, {{Javadoc:SE}}, {{Javadoc:SE-guide}}, conditional templates whose purpose is to update the links when the master template is updated.
{{ISP}}, {{Openproxy}}, {{repeat vandal}}, {{SharedIP}}, {{SharedIPEDU}}, {{s/wnote}}, not messages to the users (so it is no problem if they change from their original form), so no reason to subst, but the standard reasons not to subst apply.
{{tl}}, {{tlp}}, {{ttl}}, {{ti}}, {{tic}}, {{tls}}, {{cl}}, {{ccl}}, many very widely used, {{tl}} with over 7,620,000 transclusions at last count. Quicker to type than their substitutions, and non-substituting them allows new Wikipedians to learn about them and be able to use them.
{{sockpuppet}} and similar templates are intended to be placed on the top-level page in user space. (Note: warnings and notices in the user talk space should be substituted.)
Most templates related to renaming or deleting pages are used temporarily, and thus do not need to be substituted (it just makes more work to delete them).
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