Board Thread:Tips, Tricks, and Questions/@comment-24127824-20131103210331/@comment-174.101.218.124-20131103232726

Slots and storage. If you buy all available slots early you can go above the maximum 40 slots because of the +1 slots you gain while leveling, otherwise it caps at 40. Storage because it's great to throw good monsters in there so you don't accidentally fuse them and it reduces the slots taken up in your main bag.