An closed center, older John Deeres mostly, you have constant pressure and varible flow (gallons). A open center is the opposite, varible pressure and constant flow. The new systems are called flow and pressure compensating, which does both. This different systems are important in larger Ag tractor where there are different requirements for the hydraulics. The smaller tractors 100HP and below, the simpler open center is used, it's adequate, and is less costly to maintain.
Chuck M.