This contains the Packet Identifier from the SUBSCRIBE Packet that is being acknowledged.
The payload contains a list of return codes. Each return code corresponds to a Topic Filter in the SUBSCRIBE Packet being acknowledged. The order of return codes in the SUBACK Packet MUST match the order of Topic Filters in the SUBSCRIBE Packet.
A SUBACK Packet is sent by the Server to the Client to confirm receipt and processing of a SUBSCRIBE Packet. A SUBACK Packet contains a list of return codes, that specify the maximum QoS level that was granted in each Subscription that was requested by the SUBSCRIBE.