David,
When you build a translation based on a message type, in your case A31, for the translation this is only a template to let you choose the fields but it does not tie your translation to only A31 messages. The only way to limit what message actually uses the translation is by the route. When the translation is in use by the engine, the message will be parsed and validated based on the actual message type. In your case you where getting an A08 with out a mandatory PV1, so the HL7 parser was correctly identify your missing segment when translating the message.
John Mercogliano
Sentara Healthcare
Hampton Roads, VA
