In "Tomorrow and Tomorrow and Tomorrow" we are introduced to James Kirk, captain of the UEFEnterprise. This prefix is visible on the dedication plaque when La'an arrives on the bridge.
Curiously, later in the episode when La'an asks Kirk where he was born he says the USSIowa.
What gives? Was the Iowa was a cargo ship? Maybe in this timeline, "USS" stands for "United Shipping Service." Or perhaps at some point, UEF ships did use the prefix and they changed it because the wanted "Earth" more clearly in the identifier.
There are a bunch of ship prefixes in Star Trek for ship..."We" just mostly see Starfleet Ships. In the OS the first season had a bunch
I'd guess the United Space Ship Iowa was a "Residential ship": A ship for families to live on. Though it could also be a cargo ship, like the ones seen in ENT.