Best and easiest way to remove an outline from a part of an object ?

[Videodope.net]

Hi to all, please could somebody teach me the best and easiest way to remove an outline from a part of an object ?

Example given: I draw a rectangle, add a 3mm outline but on one side of the rectangle I don't want an outline, how to do this ?

When cutting a part with Ctrl+4 the outline adjusts to the new cut shape.


Many thanks !