Projection Entity Associations - What determines usability?

Hello Everyone,

Our company will be transitioning over to Aurena over time and I was playing around with projections in one of our test environments.  When adding standard entities to other projections it appears as if there is something that allows or disallows Parent/Child Entity Connection.

For instance, if I add the PartRevision entity to the InventoryPartHandling projection I am able to see key fields for both such as:

However, if I do the same thing by adding the ProdStructure entity to the InventoryPartHandling projection I do not get any key fields for the ProdStructure:

Is there some type of dictionary that states what entities can be added to certain projections?  Or do we have some control over this?

Thanks,

Derek Hansen