Is there a rule or a general advise or do we have to improvise in each case?
What is meta?
- Any Stack Exchange user can ask a question
- The community provides support, votes on ideas, and reports bugs
- Your voice helps shape the way Stack Exchange operates
I would normally choose support.
Which site to ask on is definitely help with one of the sites.
I guess you could also use discussion, if you want to really discuss which site it should be on.