That was after an Arbiter by the name of Fal 'Chavamee committed heresy by openly objecting the Covenant faith. This lead to him being shamed and dishonored, before being killed in combat. Consequently, the San'Shyuum hijacked the role of Arbiter, turning it into a badge of shame and making it so only the Hierarchs could bestow the mantle of Arbiter to the Covenant's most spectacularly failed Sangheili. The San'Shyuum used it for the dual purpose of removing the possibility of rebellion, and stamping out threats to the Covenant's stability.