Originally Posted by skimthetrees

Did you try multiple programs' SAUA? United and Aeroplan? United's has been known to break periodically. I am not saying that there is not another hidden inventory bucket tied to SAUA, just that cases have appeared here where no SAUA is available even with I inventory and even if there is another hidden fare class determining SAUA availability Z more closely matches it than I based on multiple reports here.

To summarize:
I does not always match.
Z does not always match (but most of the time it does and it matches in more cases than I).
Z is a better indicator than I for SAUA availability even if it is not perfect.