The difference between them is the fare bucket they are in. The fares carry similar restrictions but the booking codes at the beginning of the fare bases indicate their position in the hierarchy, thus, the amount of seats available for sale at that fare. B fares are higher than M and H fares, which are higher than E and U fares. These can be adjusted, of course, and United can opt to sell as many seats at a U fare than they do at a B fare, and do, in some lower yield markets, but in markets that have shown higher inelasticity in pricing for last minute travel or have a scarcity of service, the higher B and Y fares may apply to the very last seats available for sale.