Very basically, a good stable fast connection may connect as an ultrapeer, and less fast one probably as a leaf. Ultrapeers connect to many leafs and other ultrapeers, whilst leafs only connect to a few ultrapeers, and through them to the rest of the network. The are"shielded " by the ultrapeers.
Limewire basic leaves connect to 3 UPs and pro to 5 Ups. Apparently from what you say Frostwire now connects to 4 UPs which is one of Frostwires tweaks
