At the RD by me, they have the small "3.5 and down's, which come 3 slabs to a cryo pack, and the mediums, which I believe are 3.5 lbs and up per slab.
To borrow from Craigs site that Larry linked...
3 1/2 & down, 4 & over, etc. This is butcher talk for the weight of a slab of spareribs. A "3...