External applications are similar to command actions initiated by an alert policy, but have these important differences:
An alert policy action is executed automatically, based on the raising, acknowledgment, or clearing of an alert; an external application is explicitly launched by a user of Operations Sentinel Console.
An alert policy action executes on the Operations Sentinel server, while an external application executes on the Operations Sentinel workstation from which it is launched.
An alert policy action always executes in the background and can have no user interface; an external application can, but need not, interact with the user in a variety of ways.