The Packet Identifier field is only present in PUBLISH Packets where the QoS level is 1 or 2
The Payload contains the Application Message that is being published. The content and format of the data is application specific. The length of the payload can be calculated by subtracting the length of the variable header from the Remaining Length field that is in the Fixed Header. It is valid for a PUBLISH Packet to contain a zero length payload.
The Topic Name identifies the information channel to which payload data is published.
A PUBLISH Control Packet is sent from a Client to a Server or from Server to a Client to transport an Application Message.