What is the type or vtype in a message argument?

The type or vtype (which is short for value type) in a message argument indicates the semantic domain in which the argument's value has meaning and is determined by your application.

Vtypes are analogous to typedefs in C. Every vtype, by convention, corresponds to only one of the three possible data types for argument values.

The vtype mechanism allows you to declare two values as the same type; for example, you could declare both the vtype messageID and the vtype bufferID as C strings with different semantics for each: some operations are valid on messageID only, some operations are valid on bufferID only, and some operations are valid on both vtypes. The pattern-matching mechanism makes sure that a request with a bufferID string does not get matched to a pattern for an operation that is only valid on messageID strings.