When connecting Flow Objects with Message Flows, which constraint is valid?

The fundamental rule for Message Flows in BPMN is that they represent communication between separate participants. Participants are represented by Pools. Therefore, a Message Flow must connect two separate Pools or Flow Objects contained within separate Pools. It cannot be used to connect objects, such as activities or events, that exist within the same Pool. Sequence Flows are used for showing the order of execution within a single Pool.

A. The angle of a connector is a stylistic choice for diagram readability and is not a rule enforced by the BPMN specification.

B. A Message Flow can validly connect a Flow Object (like a Collapsed Sub-Process) in one Pool to another Pool or an object within it.

D. This describes a valid communication pattern: a Message End Event sends a message, and a Catch-Message Intermediate Event receives it.

1. Object Management Group. (2013). Business Process Model and Notation (BPMN) Version 2.0.2. OMG Document Number formal/2013-12-09. Section 8.3.5, "Message Flow," p. 98, and Table 8.3, "Connecting Objects for Message Flows," p. 99. The table explicitly states, "A Message Flow CANNOT connect two objects within the same Pool."