"I heared that it is used to invoke(initiate) another process in occurance of an event"
That's correct.
"In this case how it is differ from chained process (i.e subprocess in ES)?"
It's similar but still different.
With the chained process, you decide when you call the sub process by adding an step your process. But sometimes, you don't want to add a step to the process. You want to rely on an event being thrown from somewhere.
The event might be thrown by one of the step in the current process. For example if you use the user service, you can catch the OnDeadline event and then start a new process when a deadline occurs. You can't do that with a chained process, since the item doesn't really complete the user step yet.
Sometimes you might need to catch an event that is not coming from your process at all. For example, LC Rights Management can throw an event when a document is being viewed in Acrobat. With event you can start a process when that happens.
Jasmin
