If my PUT passes validation, I'd expect the resource to be overwritten with my request body (#2). If it doesn't, 400 (#1). If I do a PATCH, I'd expect #3. If P

If my PUT passes validation, I'd expect the resource to be overwritten with my request body (#2). If it doesn't, 400 (#1). If I do a PATCH, I'd expect #3. If PUT falls back to PATCH, then there's no replace action, which might be useful. It doesn't end up being a huge deal practically, because partial updates are probably more common than replaces, but that's my reasoning for it.