I'm new to the site, I don't know if it's okay to answer.
My answer is addressed to Bart:
The SA1 part must remain there, it is a welded component in sub-assembly 1.
Part A1 is in the final assembly (assembly of welded subassemblies and independent parts) and must remain there, it is an assembled component that is itself positioned in relation to a part of the subassembly2.
Moreover, when I go down into the SA1 to constrain the part at this level by copying the dimensions taken in the assembly (since there the part is in the right position) SW replies "this constraint cannot be solved".