Final answer:
Routing a message to a sub flow means the message gets processed by a reusable sequence of steps within the calling flow's context, and it runs in the same thread as the calling flow without its own exception handling.
Step-by-step explanation:
When you route a message with a flow reference to a sub flow in MuleSoft or other integration platforms, the message is passed to the sub flow for processing. A sub flow can be considered a reusable sequence of processing steps that are executed within the context of the calling flow. The sub flow receives the message, processes it according to its configured components, and then returns to the calling flow once processing is complete. Since the sub flow does not have its own exception handling, any exceptions thrown are propagated back to the calling flow. It's also worth noting that sub flows run in the same thread as the calling flow, unlike private flows which can run in different threads if asynchronous processing is used.