Danfoss / Eaton-DR SHF (MD)

Bolted onto the rear of the primary piston pump, it gives you a mechanical connection point for mounting an auxiliary pump or other accessory. Instead of running a separate drive line, you tie your secondary unit right into the back of the main pump through this shaft. It's a pretty common approach when you need to run two functions off one power source without complicating the rest of the drivetrain.