This read-only octet string, up to 15 bytes, reports the SAPI actually received in this OTU4 (about 111.8 Gbit/s) line facility's trail-trace overhead, compared internally against the expected SAPI to detect a mismatch. Operators read this directly when chasing a TIM alarm, to see exactly what the far end is really sending rather than what was configured as expected. For example, if a TIM alarm fires on the OTU4 facility on slot 3, comparing this received value against the expected SAPI often reveals a simple provisioning typo at the far end.