Setting up the MUS (Multiple Unit Synchronization) ID Jumpers on Cummins Marine engines allows the CAN network to properly identify the Starboard and Port engines (which are termed MASTER & SLAVE respectively).
The MUS ID jumper is a 4-pin Deutsche plug located somewhere on the on-engine harnessing. The connector will have a looped wire that tells the engine whether its the master or slave. When pin positions 1 & 2 are jumped together, the engine will be setup as the MASTER (typically the STBD engine). To setup an engine as the SLAVE engine (typically the PORT engine), wire positions 1 & 3 need to be jumped together.