This enables or disables sending a 10 MHz frequency reference signal from the modem to the BUC over the coax cable. Many BUCs need this external reference to lock their internal frequency synthesizer accurately; disabling it is only correct if the BUC has its own internal reference, otherwise the actual uplink frequency will drift. Leaving this disabled(0) on a BUC that requires an external 10 MHz reference would cause the transmit frequency to drift from the configured value, potentially interfering with an adjacent carrier.