Protocol body represents particular payload on transport level (OSI layer 4).
Typically this payload in encapsulated into network level (OSI layer 3) packet, which includes "protocol number" field that would be used to decide what's inside the payload and how to parse it. Thanks to IANA's standardization effort, multiple network level use the same IDs for these payloads named "protocol numbers".
This is effectively a "router" type: it expects to get protocol number as a parameter, and then invokes relevant type parser based on that parameter.
This page hosts a formal specification of protocol_body using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing library.