This ABS 3" female NPT fitting is primarily used as an outlet for your black tank.  It is solvent welded on to your tank. Its threads protrude outside the tank.  It allows you to then thread a MPT fitting (like the Valterra MPT valve mounting flange) into it.  It will create a water tight seal and can be removed in the future if necessary.

This fitting cannot be solvent welded to milky white polyethylene tanks.