This won’t be the best explanation, but here goes. When you press P you create a new mesh object, when you press Y it is still part of the original object. The easiest way to see it is to try it yourself,
create a cube
go into edit, subdivide the cube
on one section use P and another use Y, you can no longer modify the P one, but you can the Y one
go into object mode, move and scale the P one and nothing happens to the Y
also look in the outliner tab area, the P created a new object and the Y doesn’t.
Hope this helped.
