Say I have the following assembly structure with external relations (shown by ->):

• Top Level Assembly
• Sub Assembly 1
  • Part 1
  • Part 2
  • Part 3 -> (Part 1 & 2 within Sub Assembly 1 )
• Part 4

Part 3 has an in-context external relation within Sub Assembly 1 which relates it to Part 1 & 2. With this setup, it wouldn't be legal to then edit Part 3 in the context of the Top Level Assembly and make an external relation