Both Referral A & B are special declines that mean 'refer to the issuing bank'.
Referral A is the same as a Pickup response from the cardholder's issuing bank.
Referral A (or Pickup) means that the card has been marked as lost or stolen by the cardholder. The cardholder should speak with their issuer to discuss this further.
Referral B means that the bank requires more information before they can approve the transaction. This is only relevant for Virtual Terminal (Telephone & Mail Order) transactions, and in the eCommerce environment, it's treated as a decline. Typically, when a merchant receives a Referral B, they should contact their acquirer as this requires manual authorisation (a code is provided) in order for the merchant to complete the transaction. This is because the issuer is refusing to automatically authorise the payment.