The definition of practical jewelry, a Rosarius is a small amulet, design-wise often in keeping with the Imperium's eagle and skull fetish, that acts as both a symbol of authority for high ranking members of the Ecclesiarchy, and personal defence, as it projects a conversion field that converts would be lethal blows into flashy light shows. That last point is particularly important as it was used by The Universe's Most Evilly Named Man, Goge Vandire, to trick the Brides of the Emperor into believing that he was protected by divine intervention, sparking that whole Reign of Blood thing, oops.
As the Imperium is an insular cliquey clusterfuck of a bureaucratic organisation where no-one can play nice with one another, the incredibly useful Rosarius is hoarded by the Ecclesiarchy and only given out to those they deem zealous enough. So the next time you're sitting around wishing your squishy 'umie could have an invulnerable save, you know who to blame.
- Space Marine Chaplains are given one as part of their 'welcome to being a priest of the Emperor' starter pack along with the fancy hitting stick and sent on their merry way. The Wolf Wolves get a Wolf Amulet for their Wolf Priests because they are special.
- As the not quite military arm of the Adeptus Ministorum, SoB Canonesses get to keep their un-helmeted heads safe from rogue shrapnel without having to muffle the manic street preaching during battle.
- Despite having very little special about them, Priests, Confessors and Missionaries get them for the same reason. I suppose the Ministorum wants to protect their investments as apparently people hopped up on the worship of the Emperor are less common than ancient technology.
- Although the Inquisition on the whole in theory doesn't have access to them, some Inquisitors either have friends in the right places who would gift one over as a token of appreciation, though this is more likely for Witch Hunters due to how close they work with the "not-Men-at-Arms of the Ecclesiarchy. Sometimes a particularly brazen inquisitor might requisition one from the corpse of a fallen priest, and as they almost always have the highest say-so in the room, they might just get away with it.
- Interrogator-Chaplains of the Dark Angels have a tradition of blinging out their Rosarius (
Rosariuses? Rosarii?Rosaria how did he make it out of Schola Progenium?) with black pearls from the planet of Malmar, a damp Death World close to the fringes of the Eye of Terror. One of said pearls is awarded every time they make a member of the FallenREGULAR GARDEN VARIETY FILTHY HERETIC WHO HAPPENS TO BE VERY DANGEROUS repent for their sins.