In addition to the Chapter, Council, and Commandery, there are other Masonic bodies whose memberhsip is predicated on membership in the Royal Arch or other/all of the York Rite. These are styled "Appendant Bodies".
They are invitational in nature. Candidates must be members of the York Rite or one of its bodies, be proposed by a member of the body, pass a bollot for membership, and then be invited to membership.
Like most invitational bodies, members represent the more dedicated and select members of the fraternity. This selectivity is generally expressed in the limitations of the body's membership size, selection processes, and requirements.