Multiple backups, because I may not know exactly when BadThing(tm) happened.
I can go back to whichever image works.
Full + Differential: Full is Monday, Differentials are whatever happened on Tuesday, Wednesday, etc, etc.
To reconstitute, you need the Full, and whichever days Differential you want.
'New Every Week' and 'Keep two weeks' means you have 2 Fulls, and whatever Differentials have happened in the interim. It deletes the previous (oldest) Full
Today is Tuesday. You make a Full on Mondays, and Differentials every day.
If you want to go back to the way the system was last Wednesday, you need the Monday before that, and the following Wednesday of that same week.