I think you are supposed to use SUSER_SNAME() in most situations.
i believe the difference is SUSER_SNAME() would return a domain\username for those people who are not explicitly on the server, but only get access through an inherited role;
so if you grant a windows role access, SUSER_SNAME() would return results, but SUSER_NAME() is blank, because that user is not explicitly in the sys.server_principals table, right?
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