A 'circle' in the Ministry of the Scroll, is the term for a level of study in a field. It is meant to encompass all the teachings required to have a specific level of knowledge.
The term comes from the circular study rooms, where students would be said to sit or stand in the circle together.
Thus a circle of students was a level of students and so the etymology developed.
A circle can be of varying length, and contain a variable amount of content.
A circle of warriorship or priesthood can be earned for 6 months of basic training, or 2 years tour of astral space aboard the Gith ships, for example.
A member of the church is forbidden from practicing both circles simultaneously. Practitioners are tested and graduation from a circle allows one to seek another, from either 'ladder'.