Since astronomy is within our scope, most things that would fit on Astronomy SE also fit here. If they are asked here, they should not be migrated. There are probably a few corner cases that are off topic for us and on topic for them, but those are rare.
For other topics, in principle it's straightforward: a question which is about physics (or astronomy) is on topic here, one which is not about physics (or astronomy) is off topic here. Of course, there can be some debate about whether something is about physics/astronomy or not, but again, those ambiguous cases are relatively rare.