The CPU module is one of many modules which can be installed in the Universal Relays series; other modules include power supply, contact input and output, CT/VT, inter-relay communications, and transducer input and output. The series contains lengthy product numbers due to the fact that each segment of the model number corresponds with one of these modules; segments with an “XX” indicate that the module is not installed in the Universal Relay on arrival while segments with a number and letter indicate that it does come with the module. Users can reference the order code tables found in the Universal Relays product manuals for more information.