Qualifying function parameters using return vs return scope

[Per Nordlöw]

Can somebody explain when

- a free function parameter or
- a struct/class member function's `this`-parameter

should be qualified as `return` vs `return scope`?


Is there a part of the spec that explains this difference?

Further, are there differences in the way

- a free function parameter,
- a `struct` member function's `this` pointer, or
- a `class` member function's `this` pointer

is handled?


I've noticed cases where using the `return scope` instead of just 
`return` prevents -dip1000 from detecting invalid escaping of 
scoped objects. Is this intended or a bug?