The traditional Christian hierarchy of nine angelic orders arranged in three triads, a system that strongly influenced medieval and later sacred imagery.
The Nine Choirs of Angels are traditionally arranged as Seraphim, Cherubim, and Thrones; Dominions, Virtues, and Powers; Principalities, Archangels, and Angels. The hierarchy was systematized in the Christian tradition by Pseudo-Dionysius and became deeply influential in medieval theology, liturgy, poetry, and art.
Etymology
Derived from the angelic hierarchy associated with Pseudo-Dionysius the Areopagite; choir from Latin chorus.
Sculptor Notes
For the sculptor, the hierarchy matters because different orders may be represented differently. Seraphim and cherubim may appear as concentrated winged forms, while archangels and angels are more often fully anthropomorphic. Knowing the order prevents the indiscriminate use of every winged figure as the same visual type.
References
https://www.metmuseum.org/exhibitions/listings/2018/heavenly-bodies/exhibition-galleries-met-fifth-avenue