A transaction is "COMPLETED", if it directly has been completed by the system because the sender has sent the whole and exact coin amount of the payment order, as requested.
A transaction may be "ACCEPTED" by the merchant manually only if it is a refundable transaction. The status "refundable" indicates that the user has sent a lower than the required amount. In other words, the sender made a partial payment.
Any refundable transaction might be rejected or accepted upon the merchant's preference. Once the merchant accepts the transaction and doesn't refund it, then it is "accepted".