Topic Filter indicating the Topic to which the Client wants to subscribe. The Topic Filters in a SUBSCRIBE packet payload MUST be UTF-8 encoded strings. A Server SHOULD support Topic filters that contain the wildcard characters. If it chooses not to support topic filters that contain wildcard characters it MUST reject any Subscription request whose filter contains them
This gives the maximum QoS level at which the Server can send Application Messages to the Client.
The payload of a SUBSCRIBE Packet