The HTTP message payload body is the information ("payload") part of the data that is sent in the HTTP Message Body (if any), prior to
transfer encoding being applied. If transfer encoding is not used, the payload body and message body are the same!
For example, in this response the message body contains only the payload body: "Mozilla Developer Network":
HTTP/1.1 200 OK Content-Type: text/plain Mozilla Developer Network
By contrast, the below response uses transfer encoding to encode the payload body into chunks. The payload body (information) sent is still "Mozilla Developer Network", but the message body includes additional data to separate the chunks:
HTTP/1.1 200 OK Content-Type: text/plain Transfer-Encoding: chunked 7\r\n Mozilla\r\n 9\r\n Developer\r\n 7\r\n Network\r\n 0\r\n \r\n