How to send messages within a pool in a BPMN model?

Connecting between different pools, there are message flows. However, I don’t understand how to exchange messages within a pool (Department A & B).

Specifically, I want the finance department to be notified ahead of time by logistics if no internal purchase order has been created for the scanned delivery bill. The goal is that the finance department is informed very early (by message), so that afterwards processes within the finance department do not have to be stopped.
any Suggestion?

Sending messages within a pool is not supported by the BPMN spec.

If departments interact via messages they are separate pools, running separate (dependent) processes.

This is what is laid out by the BPMN specification. Do not quote me on a particular line though :wink:.