The ordered hierarchy of angels developed in Christian theology and traditionally divided into three hierarchies containing nine angelic orders.
Celestial Hierarchy is the traditional Christian ordering of the angels into graded ranks. The system most influential in Western art comes through Pseudo-Dionysius and was elaborated by medieval theologians including Thomas Aquinas. It arranges nine angelic orders in three hierarchies.
The hierarchy provided artists with a conceptual structure for differentiating heavenly beings rather than treating every winged figure as interchangeable.
Etymology
From Latin caelestis, “heavenly,” and Greek hierarchia, “sacred rule or order.”
Sculptor Notes
For sculpture, hierarchy should be made visible through attributes, position, scale, gesture, and degree of abstraction. A seraph surrounding a throne, an armored Michael, and a messenger Gabriel solve very different formal problems even though all belong to the angelic world.
References
https://www.vatican.va/content/catechism/en/part_one/section_two/chapter_one/article_1/paragraph_5_heaven_and_earth.html; https://www.newadvent.org/summa/1108.htm