How to custom encoding?

I have managed JSON and raw encoding of messages to an SUT adapter. This example has custom encoding, how is that normally implemented? I know how the packet should be laid out and structured, but not sure how to encode/decode the message to send over UDP to SUT.

With the raw encoding I did, I had to specify the HEXORDER and length of all the hexstring fields, then manually manage all the unions in the c++ port receive functions to correctly generate incoming messages.

What is intended by the use on non standard encoding?

type record Message { union { Message_Request_TYPE msg_Request, Message_Response_TYPE msg_Response } content } with { encode "MRP"; }