This is the ICAO doc.
This is a summary on the PPL/IR website.
I am unclear about what Block 18 codes one is supposed to use in the GA context. For example anybody with a GPS and a coupled autopilot could put in RNP 1 and probably RNP 0.3.
I can see this being used by the Eurocontrol computer as yet another reason for rejecting flight plans, by matching obscure declared equipment capabilities against State-declared airspace requirements.
I am convinced that if the number of people whose sole purpose in life is to produce bollox regulations was reduced, aviation would have to collapse, out of respect.