Prop Challenges (Levels 1-3):
Users may only have one prop challenge (Levels 1-3) at a time of any given size. For example, you can only have a single 100K challenge at a time, but you can have another challenge of a different size.
Prop A-Book (Level 4):
Users may have multiple A-Book accounts, even if they are of the same size, at any given time. For example, if you reach Level 4 with a 100K account, you can then purchase a new 100K challenge - if you get that 100K challenge to level 4, you will have 2 100K A-Book Challenges at the same time.