The DTC U0073 OBDII code refers to the communication circuit that exists between the different control modules of the vehicle. This circuit is called Control Area Network (CAN).
The vehicle’s controller modules can communicate and exchange information with each other because of the CAN bus. When trying to diagnose this code, maybe an automotive scanner may not be able to communicate with the vehicle depending on the affected circuit. The “A” designation may refer to the high-speed CAN bus or low-speed CAN bus, depending on the manufacturer.