A system must implement the required protocols.
A system should implement the recommended protocols.
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.
These protocols are for use in limited circumstances. This may be because of their experimental state, specialized nature, limited functionality, or historic state.
These protocols are not recommended for general use. This may be because of their limited functionality, specialized nature, or experimental or historic state.