This paper characterizes messages by the constraints they place on participation of entities in processes. Constraints are expressed in two ways: informally as usage patterns of the Process Specification Language, and formally as extensions to it. The extensions reduce ambiguity and increases expressiveness compared to conventional process modeling languages, by defining the relation of the extensions to existing PSL execution concepts. Examples drawn from a spectrum of message types illustrate the range of variability in the reaction of the message receiver. The extensions are used to formalize message forwarding, a useful technique in specifying composable processes. These suggest simpler alternatives to formalize inputs and outputs.