RotoPaint - Nuke 101: Professional Compositing and Visual Effects - page 242

Paint is simply a way to paint on the frame using brushes and similar tools we've all
grown accustomed to, such as a clone tool (sometimes called a stamp tool). This tool is
rarely available in a compositing system as a way to paint beautiful pictures, but rather
to fix specific problems such as holes in mattes and or spots on pretty faces and to draw
simple, but sometimes necessary, doodles (I can't really call them paintings).
Note
Up to version 6, when RotoPaint was introduced, two separate tools served
these functions. One was called Paint and the other called Bézier. They are
still available (although hidden) in Nuke for backwards compatibility.