Yamaha MTX Series Setup Manual page 10

2.
Specify the number of units that will be connected in your MTX/MRX System, and click
[Next>].
In "YDIF Connected," specify 1 as the number of MTX3 units.
3.
Verify that the MTX's UNIT ID is 1, and then click [Next>].
Unless you have specific reasons for doing so, use the UNIT ID that is assigned.
Example 1) Basic MTX3 system example (analog connections)
10
MTX Setup Manual