This flag identifies whether the circuit's current activation represents a call to a designated backup PVC rather than the primary path, letting the admin distinguish 'this circuit is up on its normal route' from 'this circuit has failed over to its backup' directly from the circuit table without cross-referencing a separate backup configuration. For example, an admin noticing cktCallingBackup reading true(1) on a circuit that's technically operational would still flag it for follow-up, since the primary path it's supposed to use is presumably down.