There classes are descendants of Network::PacketParser class.
Network::Receive and Network::Send are parser helper classes. They contain parser helpers which serve as a workaround in the absense of the capable parser subsystem.
Network::Receive and Network::ClientReceive are message handling classes. They contain built-in handlers for messages coming from the server (Network::Receive) or from the client (Network::ClientReceive), which store information from network messages to be used later in other modules (like the AI).
Network::Send is the message sender class. It encapsulates network messages into simple, easy-to-use functions to be used outside of network subsystem.
Any descendant ServerType class may customize message handlers and parser helpers, but they should refrain from that other than for servertype-specific features and debugging.