This read-only octet string (up to 15 characters) holds the Source Access Point Identifier (SAPI) that an OTUC3 (300 Gb/s-class flexible OTU) trunk port is configured to expect in the incoming trail trace overhead. An administrator relies on this to confirm the near end has been provisioned with the correct expected origin identifier, since a mismatch against the received SAPI raises a Trail Trace Identifier Mismatch (TIM) defect. For example, if this expected SAPI is set to 'SITE-A-TX', a receiving OTUC3 node will flag a TIM defect the moment frames start arriving with a different SAPI value.