Hi! The reason why Derive isn't available in the context of an assembly is that Derive isn't a suitable workflow in such situation. Technically, Derive is out-of-context geometry referencing. This means, the derived source and the derived part aren't aware of their context. Certainly, the shape change in the source does push to the derived part. But, it is limited to the derived objects, irrespective of how the geometry was changed or where the change happened. Derived part can stand by itself.
In order to borrow geometry from another component within the same assembly, Adaptive is required. It can be projecting edges, the face loops, or copying the entire body geometry from another component within the assembly. Such relationship only makes sense within that particular assembly, not elsewhere.
Many thanks!
Johnson Shiue (johnson.shiue@autodesk.com)
Software Test Engineer