USAT value in the kIRG_USource property

[drmccreedy .]

I see one codepoint now has the kIRG_USource property value of "USAT" in
the Unihan_IRGSources.txt file from Unihan.zip:
   U+20991 kIRG_USource USAT-00061

UAX #45 (U-source Ideographs, http://www.unicode.org/reports/tr45/index.html)
mentions UTC and UCI but not USAT.

UAX #38 (Unicode Han Database, http://www.unicode.org/reports/tr38/)
updated the syntax for the kIRG_USource property (but not the description)
to U(TC|CI|SAT)-[0-9]{5} so I'm pretty sure it's not a typo.

Where can I find a description of the USAT value?

Thanks,

David McCreedy
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