This doesn't mean Priesthood is closed off to Friars, once they've fufilled the requirements, it just encourages members to stay in various Orders.
Orders themselves generally have their own hierarchy as well. This is more apparent in Military Orders, where Sgt-brothers are ranked below Knight-brothers, which are all under the Grandmaster.
To say a Friar is a "lower rank" then a Priest is again comparable to enlisted vs officers in the military. What does socially inferior really mean anyway? Can't attend cocktail parties?
