This read-only octet string (up to 15 characters) holds the trail trace identifier (TRC) that an STM-64 (10 Gb/s SDH) client port expects to receive, the SONET/SDH equivalent of the SAPI/DAPI trace check used on OTN facilities. An administrator uses it to confirm the correct expected trace string has been provisioned, so a genuine mismatch against the received trace reliably indicates a misconnected or unintended path. For example, if STM-64 unexpectedly stops matching this expected trace value, that mismatch is what an operator uses to suspect the fiber has been re-patched to a different far-end port.