You can use whatever configuration of blue racks best holds your stuff. I use the bottom rack with the ring bottle insert because all of our neb cups and Emily's Aerochamber kind of stand in the rings rather nicely... her neilmed & ayr bottles fit that way too (I stand them upside down.) I use the upper tray for the small parts (mouthpieces, little round pieces.) So in our case, there is one extra blue tray I opt not to use ...but the others are helpful.