EventEmitter uses the EventEmitter class from Node.js. It allows triggering and handling events from anywhere in a mobile or reactive screen.

OutSystems events triggered by a block can only travel 'up' the component tree. They can only be handled by parent blocks, and need to be passed on by every block if they need to go up a couple of levels.
For 90% of the usecases this is fine and still what I would recommend to use.

However, sometimes we want to handle events by blocks that are NOT a parent block of the block that triggered the event. In that case the EventEmitter is useful. It allows triggering events anywhere in the screen and handle them anywhere as well.