It would be really useful in Inventor if you could have a flexible sub assembly, which could then be part of a contact set within a higher level assembly (GA).
At the moment, you can make a sub assembly flexible or part of a contact set, but not both at the same time.
The workaround is you have to re-structure your assembly, promoting the parts in the sub assembly to the top level GA, where you then don't need to make them flexible, so they can be part of a contact set. The only problem is now you have blown away your assembly structuring for BOM/Parts List purposes.