There are no specific rules AFAIK (might be some guideline for the moderators). It makes sense to post a question in the forum where most likely a correct answer can be given.
It shouldn't have been posted in SD because it has nothing to do with it. And in the general ABAP forum there is not a big chance that someone with specific IS experience would answer this post. It will most likely just "drown" there.
The question is not about some general development challenge, it's about functionality that is specific to IS. Such questions usually get better response in the corresponding IS forums.