What we expect is that Part be the root part of the weld. That is, regardless of the size of either of the parts involved, Part1 is moved to match onto Part0, not the smaller part moved to the larger part.
Incorrect behavior can be seen by placing a weld on a small part and setting it’s Part1 to a large part. The small part moves even though it is Part0, the root part. The root part should not move. Part1 should move. This is what we expect.