This read-only octet string (up to 15 characters) holds the trail trace identifier (TRC) that a 10G WAN PHY client port framed as SONET 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 10G WAN SONET 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.