It is not possible to define the on-topic rules of any forum such that they only include those subjects for which that forum is the best available resource. For one thing, any forum that wants to grow has to attract new regular contributors and as it does so, it may become a better or even the best forum for particular questions. So really, there are going to be four categories of questions for any forum:
- On topic -- best place to ask.
- On topic -- good place to ask but there may be better.
- On topic -- not currently a great place to ask because we don't have a lot of people with expertise in that area.
- Off topic.
The only place for a forum to broaden it scope is in the third category. If we cut it off, for instance, by declaring FrameMaker question off topic while Scrivener questions are considered on topic, that makes it impossible to grow into that area.
If the forum does not want to grow into that area (this site my decide to drop tech comm from its scope if the proposed Technical Communication SE reaches beta) then by all means make those questions off topic. But if it want to grow into those areas it cannot rule questions in those areas off topic.
How should we handle the third category of question, then? Don't rule them off topic but be frank that there are better places to ask, at least until more people show up who are qualified to answer them -- which is presumably what we want.
Alternately, we could rule all tool questions off topic, including Scrivener.