Relay SB is normally operated.

When a connection is to be established between a seized Incoming or Own-exchange Supervisory Relay Set and a Subscriber's Line Circuit, that Supervisory Relay Set applies negative battery to its A-lead, operating access relays TA-TD.

Operation of the access relays allows the C-Switch Selector to determine which links are free, and to select and mark one of them. (Free links have a positive potential on the H wire; engaged links a negative potential.) 

The 'mark' is a +22 V zener potential applied, via the C-Switch Selector, from the B-Switch Selector and operates the crosspoint relay which is common to the Supervisory Relay-Set and chosen C-D link.