This read-only octet string (up to 15 characters) reports the Source Access Point Identifier actually being received right now on an OTUC3 (300 Gb/s-class flexible OTU) trunk port, as opposed to what was configured as expected. An administrator compares this live received value against the configured expected SAPI to diagnose a TIM alarm or confirm a newly turned-up circuit is receiving trace from the correct origin. For example, if OTUC3 shows a received SAPI reading 'SITE-B-TX' when 'SITE-A-TX' was expected, that mismatch is the direct evidence used to track down a fiber patched to the wrong far-end site.