I flagged this question https://superuser.com/questions/566692/restoring-deleted-ipa-files-in-itunes as Off Topic because I felt it was more about iPads and belonged on Ask Different. The flag was rejected and I'm fine with that but just trying to understand where the line is?
The default policy is to leave the question on the site where it was originally asked, unless it's off-topic there and would be on-topic elsewhere. It's up to the user to select the site where they think their question will fit best, and to tailor the question to that site's audience and topic when asking the question.
In this case, because the question is asking about how to do something with iTunes on a PC, that is interacting with an external device, it's valid and on-topic (see the FAQ). If the question were entirely about the iPad and not the computer that it's interacting with, it would be off-topic and thus migrated to Ask Different.
There is no specific "line". Just because a question is on-topic on Ask Different, doesn't mean it's off-topic here.
There are some lines: pure iOS questions (i.e. not questions about connecting iOS devices to a computer) don't belong here, while non-Apple questions don't belong there. However, that leaves a big overlap: any questions about Macs, or any other Apple product in the context of a regular computer (including all iTunes questions, and hooking an iPad to a computer) are on-topic on both sites. In that case, you respect the OP's decision about what site to put it on; the only time you'd move it is if the OP flags requesting a move.