SYS$OPTIONS in audit file?

When audit_trail=OS, what does SYS$OPTIONS represent in the audit log file? (AKA "ACTION" is a reference to AUDIT_ACTIONS).

For example, in some tests after "audit system audit:", then "audit user" followed by "noaudit user", I see SYS$OPTIONS: "16" in the audit log, but have no idea how to relate this to anything I can look up. It must mean "user" somehow, because the information that noaudit was done is there and other typical audit trail information, but there is nothing to indicate that it was "noaudit user".

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