This selects which clock source this card uses to generate its own transmit timing: the chassis's primary system clock, secondary system clock, a fallback combination of both, its own free-running oscillator, or specific E1 line-derived references. Setting this to free-running-clock(4) deliberately decouples the card's transmit timing from the chassis-wide synchronized clock, which is appropriate for isolated test scenarios but, if left set that way in production, causes the card's outgoing signal to drift independently of the rest of the network and generate slips on circuits terminating elsewhere. For example, a card mistakenly left on free-running-clock(4) in production shows steadily accumulating timing slips on its circuits because it never syncs to the chassis's disciplined system clock.