Well...
A permanent red is permanent, whilst the 20 minute means that the player comes off permanently, but can be replaced after 20 minutes.
Essentially only the FPRO can issue a 20 minute red, and only the ref can issue a permanent red - which is all kinds of wrong.
Basically the ref makes a decisions, definite yellow, definite red, or borderline. If borderline it goes to the FPRO who then decides, but doesn't have access to a full red. But the deliberation happens whilst everyone else gets on with playing/watching some actual rugby, rather than spending 8 minutes watching endless replays at different speeds.