The optional Event Type parameter, when present in an alarm notification, classifies the alarm into one of the five basic categories specified in ITU-T X.733.
This is consistent with the ITU-T X.733 definition.

Value is one of the following:
- "communicationsAlarm",
- "environmentalAlarm",
- "equipmentAlarm",
- "processingErrorAlarm",
- "qualityOfServiceAlarm"

Readable by NMS? Yes
Writeable by NMS? No
Invariant? Yes
