Imported from previous forum
[ original email was from don mcwhirter - don.mcwhirter@easdaq.com ]
In the FIX Protocol 4.2 spec it states that 'The formats of all valid messages are detailed in the sections ‘Administrative Messages’ and ‘Application Messages’.
Within each message specification is shown a number of fields - some required, some optional.
If I compose a message and include another valid FIX tag which is not explicitly included in the message format spec is my message still a valid FIX message?