The Circle of Steel is a faction within the Brotherhood of Steel meant for dealing with internal conflicts. Formed by hardliners in the Brotherhood of Steel, who believed the rest of the organization has drifted from the original principles of preserving tech. The Circle of Steel, in terms of their position relative to the Brotherhood proper, are the Internal Affairs of the whole Brotherhood of Steel.
↑Chris Avellone: "for me, they weren’t evil, they were just hardliners who thought the rest had drifted from the original principles of preserving tech and policing their own rogues – kind of like Internal Affairs"
↑Patient Log Y-17.0: "This is Christine Royce. Knight of the Brotherhood of Steel... the Circle. Not going to make it through this, hope someone finds this message, gets it to the Brotherhood in the West. Tracked a rogue Brotherhood Elder, Elijah, here, to the Big Empty. Place is more than it seems, there's a crater hidden deep inside. Junkyard of Pre-War labs scattered across the crater's surface, all still running... like this one. Elijah's dissecting these centers, one by one... tracked him to an old Chinese-American internment camp. Survivors, Ghouls, have bomb collars. Robots moved in when I tried to intercept him... Elijah sent the camp Ghouls against us both, like walking bombs. Got hit by the explosions, woke up here. Guess the medical robots were programmed to bring wounded victims from the camp to this center. Some kind of Auto-Doc prototype lab... manned by corpses trapped inside suits that keep them moving, no idea why. Not sure how long I'm going to last... cut open my head like a lot of the humans I've seen here, feel strange, can talk, but can't hack the term... <sound of an explosion> ...wait, an explosion outside. Someone's here... someone's... <static cut off>"