The Controller Area Network (CAN) is the circuit generally used by control modules to communicate with each other. It is through this circuit that information and various vehicle operating commands can be exchanged. Also, it is through the CAN bus that information can be obtained from the car with the use of a scanner.
The BSCM (Brake System Control Module) is a controller that receives information from different sensors, through which the base brakes can be activated, although it is also used as a combined base/anti-brake system. It is worth mentioning that this system is very common to see in hybrid cars, but certain manufacturers consider it suitable for various applications.
If the Brake System Control Module (BSCM) fails to communicate with another control module that requires such information, a DTC will be set that serves as a warning of the situation. In the case of the U0129 OBDII code, it indicates that the communication from that controller could not be established.