I’m pretty sure there is a wiki for this somewhere but am unsure what the search term is to find it? ping @matthias
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It’s here: Discourse Admin Manual § 1.4. Topic Templates 
In general: when search does not work, you can find the right manual and look through the table of contents. I organized all the documentation so that every bit of information is available directly or with a link from a manual. A manual is a topic tagged manual, also marked with a 
and listed at the top of one of the following categories:
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