Thing Alarms are based on the event-triggered when a specific threshold is met. The Triggers are the listeners which consistently listen for the threshold to alter the state of an alarm. In the following figure, an alarm is triggered when the fuel level reaches one quarter, half or full tank.
Alarms are defined in the Thing definitions. You publish alarms either by publishing directly from a thing(see Publish Alarm) or use a trigger that listens to the data and triggers an alarm when required ( see Triggers).