1)BABY back ribs are generally 2lb (or maybe
2.5lb) or smaller per rack
2)LOIN back ribs are bigger, say 2.5lb and up. BOTH have "rounded" ribs
SIZE in this case determines their "name"
3)spares are usually 3-4-5 lb per rack and the bones are flat (not curved like backs)
As said, there are various ways they are cut/sold so....
speaking, back ribs are higher up on the side of the pig and spares are lower down BUT they are in one piece while still on the pig.
Spares have more flavour to them and can be tenderer than backs due to the extra fat and cartilage. A lot depends on how either kind are cooked though.
Backs are much leaner and in some peoples opinions are easier to eat as you don't have to work around any cartilage to get the meat off.
I never buy beef ribs as ours are usually skinned to the bone and have very little meat on them.
But as said, when I prepare prime rib after cooking, I'm the one who gets the bones 😁