This read-write object selects the primary timing/clock source for the AUM (ATM Uplink Module) card: an internal oscillator, an inband clock recovered from the AUM's traffic, or an external clock input. It determines where the card derives its primary reference timing from for synchronizing outbound transmission. An admin synchronizing a shelf to a network-wide timing source, such as a building integrated timing supply, would set this to externalClock rather than relying on the card's own internal oscillator.