This indicates the desired backup clock source, internal or loop-timed, that this port's card falls back to if it is configured for external clock source mode and that external reference is lost. Having a defined backup source matters because a card left with no valid clock reference when its external timing fails will drop synchronization across all its ports, not just this one, so this setting is really about protecting the whole card's timing integrity via this configured fallback. A card normally timed from an external BITS reference that is set here to fall back to looptimedClkBkup would, upon losing that reference, start deriving its clock from an incoming line instead of free-running on its own internal oscillator.