8.4.2. Credit Allocation Modes

In credit mode, credit must be explicitly re-issued by the subscriber before the broker can recommence sending messages

In window mode, the credit is automatically reissued for received messages. In this mode, the client indicates that a message has been received by informing the broker of the completion of the transfer. Though completion is essentially a form of acknowledgment, it should not be confused with acceptance which is an confirmation of ownership transfer.