Is there a document describing how the protocol work?

I am reading the source code of counterpartyd, and in the ,there is a const

    # Counterparty protocol

I assume the ‘>I’ is a part of “Counterparty protocol”, but where is the document of “Counterparty protocol” ?
Surely it doesn’t mean that I have to summarize the protocol by reading the code?

The best place is the specs at

as far as the bitpacking is concerned, the code is the "reference" here.