I read this question, which helped, but where is the line?

Just because a question is about an Apple product, doesn't mean it is off-topic (not at all actually) just because another (more specific) site exists for that kind of question.
Then again, a question that would be more at home on another SE site is bound to be duplicated on that site at some point and duplication sucks.

I feels like, generally, the opinion seems to be to keep the question on SU.
But when should it be marked off-topic then? There has to be a certain point where it is no longer SU material, but AskUbuntu material.