There are various types of events that occur in Integration Server.i.e Session start, session end, session timeout, alarm, exception etc. 
These can be seen in Developer Event Manager. Go to Tools menu in the menu bar on the developer and click on Event Manager.

We can invoke a service when a particular event occurs in this event manager. e.g: we can invoke a particular service 

when an exception occurs in any of the IS services. This is helpfull when we want to implement custom/generic exception  handling framework.