The digital flight control system (DFCS) mode control panel (MCP) supplies runway heading data to the display electronics unit (DEU) for ILS displays. You use the DFCS MCP course selector to set the course for approach.
The DEU uses the course input and ILS deviation to calculate the airplane’s deviation from the runway centerline. The DEUs compare the course input to the airplane’s track to calculate if this is a back course approach. If the course input and airplane track are different by more than 90 degrees, the DEUs show backcourse ILS displays.