The default message acknowledgement (when the server can consider the message to have been consumed by a client) is to treat it as acknowledged as soon as it is transmitted. An alternate is to require the client to acknowledge the message explicitely. Amongst other things this offers transactional message consumption and prevents flooding the client to the point of a crash and losing a pile of messages.
To handle this the client may specify and ack header in a subscription request. The default is "auto" the alternate value (there may be more later) is "client". If "client" is used the client must send an ACK frame with the message-id from the message, which may also have a transaction header with an open transaction identifier.