How an operation uses messages
You use an operation to specify the invocation information for
a program, including the program's input message, its output message, and
its fault messages if any.
The input message of a flow
The input message of a flow is a single message
that describes the input parameters with which the flow is invoked.
The output message of a flow
The output message of a flow is a single message that describes
the output parameters that the flow returns to a program that invokes the
flow.
Fault messages for a flow
The fault message of a flow is a single message that describes
the output parameters that the flow returns to an invoking program when the
flow ends abnormally.