According to Rossler’s "The U-Boat", the final installation of equipment is done after the U-Boat is launched and then it is pressure tested before the boat can be accepted as completed product. I believe its then commissioned after the crew is assigned.

Commissioned usually means its ready of service, but all boats go through a working up process. For capital ships this could take months and months of gunnery and maneuver training etc. It helped to keep the crew quality at its best through out the war.