From OpenNMS

All events have a UEI (a Universal Event Indicator), which is simply a URI-like text string used to identify the event. They also have an associated severity, ranging from Normal for interesting, but non-problematic events, through various levels of importance up to Critical. Events can be configured to be stored permanently in the database, to be displayed or not, or ignored if of no interest. Also, arbitrary text can be attached to an event for use as operator instructions to deal with the event.