This selects the clocking method, synchronous (external reference), SRTS, or adaptive, that this port's 4-port CBR card should target when recovering the clock for constant-bit-rate traffic. For CBR services like circuit emulation, the clock recovery method on this specific port directly determines how faithfully the original bit rate is reproduced at the far end; synchronous clocking needs a shared network reference while SRTS and adaptive methods recover timing from the cell stream itself, each with different jitter and robustness tradeoffs. A CBR circuit emulating a T1 over an ATM network without a shared timing reference available would typically use adaptive clocking (mode 3) on this port rather than synchronous mode 1.