Code identifying a Transaction Set

AK201 is the transaction set ID found in the ST segment (ST01) in the transaction set being acknowledged.

Identifying control number that must be unique within the transaction set functional group assigned by the originator for a transaction set

AK202 is the transaction set control number found in the ST segment in the transaction set being acknowledged.

Reference assigned to identify Implementation Convention

AK203 is the implementation convention reference, if any, found in the ST segment (ST03) in the transaction set being acknowledged.