Code Sets
Code sets are list of values used to restrict allowed values in a field.
Internal
Internal codes are values that are specific and limited and are present within the message definition.
Examples: Settlement method, Settlement Priority
External
The purpose and value of externalizing a code set is to allow for a more frequent update of the code set by for example adding new codes in the set without impacting the version of the messages and the development cycle of the messages.
Examples: Payment status, Transaction purpose, Rejected reason
Transaction Codes
The transaction codes in the statements are also external and should follow the transaction code sets and combinations.