Very simple scenario here -
An order with lines is processed. The OrderProcess accepts in an order with its attendant lines. The Fulfillment process is called for each order line. We do not have many order lines, and the processing is simple, so we run this in parallel.
Let's look at the definition of the Multi Instance sub-process -
The Send and Receive Activities will require a Conversation to do the correlation between the 2 processes. This needs to be defined at Sub-process level and NOT AT PROCESS LEVEL. I made this mistake once too often!
Send Activity Definition -
Receive Activity Definition -
Full lab doc HERE