Objection 1. It would seem that in the one hierarchy there are not several orders. For when a definition is multiplied, the thing defined is also multiplied. But hierarchy is order, as Dionysius says (Coel. Hier. iii). Therefore, if there are many orders, there is not one hierarchy only, but many.
Objection 2. Further, different orders are different grades, and grades among spirits are constituted by different spiritual gifts. But among the angels all the spiritual gifts are common to all, for "nothing is possessed individually" (Sent. ii, D, ix). Therefore there are not different orders of angels.
Objection 3. Further, in the ecclesiastical hierarchy the orders are distinguished according to the actions of "cleansing," "enlightening," and "perfecting." For the order of deacons is "cleansing," the order of priests, is "enlightening," and of bishops "perfecting," as Dionysius says (Eccl. Hier. v). But each of the angels cleanses, enlightens, and perfects. Therefore there is no distinction of orders among the angels.
On the contrary, The Apostle says (Ephesians 1:20-21) that "God has set the Man Christ above all principality and power, and virtue, and dominion": which are the various orders of the angels, and some of them belong to one hierarchy, as will be explained (6).
I answer that, As explained above, one hierarchy is one principality--that is, one multitude ordered in one way under the rule of a prince. Now such a multitude would not be ordered, but confused, if there were not in it different orders. So the nature of a hierarchy requires diversity of orders.
This diversity of order arises from the diversity of offices and actions, as appears in one city where there are different orders according to the different actions; for there is one order of those who judge, and another of those who fight, and another of those who labor in the fields, and so forth.
But although one city thus comprises several orders, all may be reduced to three, when we consider that every multitude has a beginning, a middle, and an end. So in every city, a threefold order of men is to be seen, some of whom are supreme, as the nobles; others are the last, as the common people, while others hold a place between these, as the middle-class [populus honorabilis]. In the same way we find in each angelic hierarchy the orders distinguished according to their actions and offices, and all this diversity is reduced to three--namely, to the summit, the middle, and the base; and so in every hierarchy Dionysius places three orders (Coel. Hier. vi).
Reply to Objection 1. Order is twofold. In one way it is taken as the order comprehending in itself different grades; and in that way a hierarchy is called an order. In another way one grade is called an order; and in that sense the several orders of one hierarchy are so called.
Reply to Objection 2. All things are possessed in common by the angelic society, some things, however, being held more excellently by some than by others. Each gift is more perfectly possessed by the one who can communicate it, than by the one who cannot communicate it; as the hot thing which can communicate heat is more perfect that what is unable to give heat. And the more perfectly anyone can communicate a gift, the higher grade he occupies, as he is in the more perfect grade of mastership who can teach a higher science. By this similitude we can reckon the diversity of grades or orders among the angels, according to their different offices and actions.
Reply to Objection 3. The inferior angel is superior to the highest man of our hierarchy, according to the words, "He that is the lesser in the kingdom of heaven, is greater than he"--namely, John the Baptist, than whom "there hath not risen a greater among them that are born of women" (Matthew 11:11). Hence the lesser angel of the heavenly hierarchy can not only cleanse, but also enlighten and perfect, and in a higher way than can the orders of our hierarchy. Thus the heavenly orders are not distinguished by reason of these, but by reason of other different acts.
Source: Summa Theologica (New Advent)