Hi Grant,
It depends on whether your message is a single record or has multiple records. If it is a single record, make that field a Distinguished field in your schema (that you can reference in your orchestration). If it has multiple records, use XPath in your orchestration to query the value of that field and act upon it.
In your orchestration, set up a Decide shape with an If Branch rule that states “If value == “A””, then in that flow, you can drop in a Terminate shape to terminate the orchestration.
Best,
Daniel.