At least in the case of the Eyrie Clusters (possibly the Newts as well, I forget) the formation is used for warriors who didn't do all that well in their Trial of Position; they qualified, but not as well as was hoped and more seasoning /testing was needed before sending them on to a front-line unit if ultimately worthy, or a second-line unit if not. Warriors who did well in their Trial were sent straight to a front-line command, though.