If it's a general question pertaining to a particular
format (ePub MOBI, etc...), then it's probably best to ask in the appropriate format's subforum. If it's ebook in general (non-format specific) then the Workshop would make sense. This forum is generally reserved for stuff like "how do I do this with Sigil?", "Why does Sigil do X?" and "can Sigil do Y?" and/or "can the next version of Sigil include Z?"
Basically... if it's a question where Sigil isn't really relevant to the question
OR the likely answer, then it probably belongs elsewhere (even if Sigil is what you happen to be using when a non-Sigil-related question arises).
There will be bleed-over, of course. But just avoid shoe-horning non-Sigil-related questions in here simply because you think more (or the right) people will notice it and you'll be golden.