There isn't enough documentation here.
What capabilities exist? What capabilities can be requested? What operations depend on a capability being requested/active? Hunting Hare 06:56, 13 December 2007 (PST)
In other words, the link doesn't work any longer.
I have tried to search on the viewer's project for anything related to 'caps' or 'capabilities', but, alas, the project is way too big, and in any case, the Bitbucket internal search didn't find many relevant issues. Sure, it has found many requests for all types of caps — but not a neat list explaining what to request and how. Figuring out where such information might be hiding requires a lot of experience with the viewer source code which I obviously don't have — that was, after all, the whole point in searching here on the LSL Wiki in the first place :-)
Thankfully, the link to Tao Takashi's wonderful post is still operational in 2022 and gives a good (albeit very brief) overview of what is happening under the hood — in a very generic way, of course.
The best we have these days is the Current Sim Capabilities page (which is not complete).