The Art of Scalability (Addison-Wesley, 2015) talks about Scale Cube. Here is an adaptation of the concept in the context of messaging.
X axis scaling is having more than one message flow to process messages. That is, having more instances of the flow running.
Y axis scaling is having one message flow for each kind of message. Say one flow for one country.
Z axis scaling is splitting the message into smaller parts say instead of having both order and customer in the same message, have a separate order message, and a customer message.