The Alarm Deadband is used to prevent multiple alarm emails/messages when a variable is hovering around the Alarm Threshold.
By default it is set to 2% meaning that after alarm is triggered the variable value must change 2% from the Alarm Threshold for the alarm to become inactive.
For example: Alarm is triggered at the threshold 100 and the deadband is set to 2%. This alarm will stay active in VDV until the variable value goes below 98.
This is good to keep in mind when setting Alarm Thresholds for large numbers where 2% can become a large number so the alarm does not go inactive.