This enumerated value selects where the SONET line's transmit clock comes from: backplaneClk (from the controller card via the chassis backplane), recoverClk (recovered from the received signal, i.e., loop-timed), or localClk (a local oscillator on the card itself); the default is backplaneClk. An admin cares because this choice determines the line's timing hierarchy — using recoverClk makes this interface loop-timed off the far end, which is appropriate in some topologies but wrong in others where the chassis should be the timing master. For example, a line meant to slave its timing to an upstream carrier's network would be set to recoverClk instead of the backplaneClk default.