Canvas Size Documentation?

Does anyone know if there is documentation on the different canvas size settings? It does not appear anywhere in the official documentation that I can find.

What is the official difference between:

  • Fill Screen
  • Custom
  • Custom on Screen
  • Desired
  • Desired on Screen

I always have issues getting widgets to align perfectly (I know about anchoring and pivots), so I am trying to learn the true fundamentals, and this seems to be the only thing not super self-explanatory or documented.