There ARE a couple things you should know about these slots:
1) There are 2 slots on the middle shelf and one on the top shelf that were added last. They were meant for smaller objects- although bigger ones will fit, depending on how big they are.
2) Since they were added last, those slots won't fill until the other slots are filled. Cycling through them with "m" WILL let you choose one of the slots - EXCEPT for the one on the top shelf. For some reason that slot can't be selected that way. But you can put an object in that slot with no problem after the others are filled.