User blog comment:Master Sima Yi/Contradicting and confusing terms in the Encyclopedia/@comment-1888015-20111202140651

From what I see it as, Mentor ceased to be the Leader of the whole Order once Altair disbanded them at the end of the Secret Crusade and Revelations, from there, Mentor became a term to mean a Master Assassin that trained a lot of apprentices, and then was reformed to mean the whole order in the Modern Age when communication (Phones, email etc.) was easier to accomplish. So, the heirarchy would be Assassin < Master Assassin < Branch Leader < Leader of the Country Guild < Mentor (Through training apprentices) < Grand Master of the Order. In Ezio's time, there were only Leaders and Mentors, and Mentor and Grand Master would be merged together later. In the Modern Day (after The Fall), most of the Assassin establishments had to be abandoned, so now it's underground with hideouts and cells replacing branches and guilds.