This read-only octet string (up to 15 characters) reports the Source Access Point Identifier that an OTU2e (enhanced OTU2, roughly 11.1 Gb/s) trunk carrying an oversized client signal such as 10GbE LAN is actually transmitting toward the far end. An administrator reads this to confirm the local node is sending the SAPI string the remote site has been configured to expect, which is useful when a trace mismatch is reported at the far end rather than locally. For example, an operator troubleshooting a remote TIM alarm might read this transmitted SAPI value on OTU2e to confirm the local provisioning matches what the far end was told to expect.