Triggering Alerts 
Alert sources define which objects create alerts, under which circumstances and which alert to send. By default, the only response to an alert operator message is Acknowledge. This can be changed on the alert source.
There are three types of alert sources:
job alert sources - defines the situations in which specific jobs raise alerts
process servers alert sources - defines the situations in which specific process servers raise alerts.
AdHoc alert sources - used in job chains, for example, with the System_Alert_Send job definition.
Each alert source uses specific rules to determine:
when an alert needs to be raised
the escalation to use
the message to send