APCA number is defined as an optional value to be returned by the Get Transactions for Account API. However if the data is held by the Data Holder then the optional field becomes mandatory. If a Data Holder does not hold the data alongside the transaction values but may have the value either for processing or held in a separate part of the organisation, are they obligated to link and return this value?
An optional field indicates that a response will be valid if the field is not supplied but it is still required that the data be shared if held. In this case apcaNumber is labelled as optional and it would appear that you are not holding the data. In this case it would be valid to respond without the
apcaNumber field populated.