https://docs.natlswgr.com/docs/W1000318.pdf
Device 43/10: Select to Trip Control Switch.
This is a three-position switch with at least three contacts, one for each relay, which obey the following table:
Contact No. Position
1: Incomer 1 2: Incomer 2 3: Tie Breaker
It is required to connect one contact to each relay. This switch selects the breaker that will trip in the eventuality that all three breakers become closed, to prevent the two incoming systems to remain connected in parallel. The contacts can be either normally-open or normally-closed, depending on the asserted logic of the corresponding logic input used for this purpose.