The severities in OpenNMS are defined as follows:
This event means numerous devices on the network are affected by the event. Everyone who can should stop what they are doing and focus on fixing the problem.
A device is completely down or in danger of going down. Attention needs to be paid to this problem immediately.
A part of a device (a service, and interface, a power supply, etc.) has stopped functioning. The device needs attention.
An event has occurred that may require action. This severity can also be used to indicate a condition that should be noted (logged) but does not require direct action.
Informational message. No action required.
This severity is reserved for use in Alarms to indicate that an alarm describes a self-clearing error condition has been corrected and service is restored. This severity should never be used in event definitions. Please use "Normal" severity for events that clear an alarm.
No Severity could be associated with this event.