What is the difference between event and task messages?

I use the message flow to interconnect activities that are in diagrams / separate processes.I use an intermediate message event when this event does not generate effort/work the participant. When the sending of a message generates effort for the participant, I use an task message. Generally I use an task message rather than a intermediate message event.When you receive a message from an participant outside, you can use an intermediate message event.I hope this helps.

To complement Marcos' post, we add:

The task and event messages are very similar in their functionality (they are used to send and receive messages).

However the intermediate events are meant to be used when sending or

receiving a message does not depend on the users but on external

business events, that may or may not occur.

On the other hand, tasks are a mandatory part of the process.

Additionally, a message flow is used to connect two processes (or pools) that send and receive messages between them. Therefore, a message flow provides also a graphical aid, to easily identify where two processes interact.