USER_ID field in alert logs (also in V$DIAG_ALERT_EXT view)

Does anyone know what triggers the USER_ID field in the log.xml to be populated? The value also exists in the V$DIAG_ALERT_EXT view.

I’ve found by observing the logs that if a temp tablespace fills up, it will log the USER_ID of the problematic SQL statement causing the issue. But other than that, it appears that value is always NULL.