This up-to-16-byte octet string sets the Source Access Point Identifier the device expects to see in the TTI field of incoming OTN frames, with a zero-length value disabling SAPI matching entirely. An admin configures this to detect trail trace mismatches, which typically indicate a misconnected fiber or wrong far-end source. For example, if the received SAPI does not match this configured value, the device can raise a TIM (trace identifier mismatch) alarm.