This document lists a "requirement level" or STATUS for each protocol. The status is one of "required", "recommended", "elective", "limited use", or "not recommended".
4.2.1. Required Protocol
A system must implement the required protocols.
4.2.2. Recommended Protocol
A system should implement the recommended protocols.
4.2.3. Elective Protocol
A system may or may not implement an elective protocol. The general notion is that if you are going to do something like this, you must do exactly this. There may be several elective protocols in a general area, for example, there are several electronic mail protocols, and several routing protocols.
4.2.4. Limited Use Protocol
These protocols are for use in limited circumstances. This may be because of their experimental state, specialized nature, limited functionality, or historic state.
4.2.5. Not Recommended Protocol
These protocols are not recommended for general use. This may be because of their limited functionality, specialized nature, or experimental or historic state.