PB is a term used to provide fluid downstream, for use by other valves in series. Each valve should have PB except the last valve in a tractor hyd system, which is a 3pt valve , and it dumps all the pump flow into the tank with all valves in the neutral position.
All of the cyl fluid usage exits the valve OUT port to tank.
If there is no 3pt involved, the last valve has to have a port to pass the unused fluid to tank.