The Severity indicates the severity (importance or urgence) of the message. Severity rules for notification addresses and notification group members can use this severity to determine which addresses and members receive a message.

When setting this value you should use the same convention used by the Console: a range of 1 to 9, with 1 being lowest severity, 5 being normal, and 9 being highest severity. 

Values outside this range may cause unexpected results in the Console.

INotificationAddress.UseForSeverityComparison, INotificationGroupMember.UseForSeverityComparison