An Event Category is defined from the Edit menu in the Event Notification configuration. The Event Category is used to send any kind of information to a Column, as opposed to direct mapping of a MIM resource. The Event Category name is a string which is used as argument in the dispatchMessage APL function to route messages to a Column.
When the Event Category is defined it is mapped against a Match Value in the Event setup tab. Then the defined Event Category is used as a parameter in the APL code with the dispatchMessage function.