Objection 1. It would seem that all the angels of the second hierarchy are sent. For all the angels either assist, or minister, according to Daniel 7:10. But the angels of the second hierarchy do not assist; for they are enlightened by the angels of the first hierarchy, as Dionysius says (Coel. Hier. viii). Therefore all the angels of the second hierarchy are sent in ministry.
Objection 2. Further, Gregory says (Moral. xvii) that "there are more who minister than who assist." This would not be the case if the angels of the second hierarchy were not sent in ministry. Therefore all the angels of the second hierarchy are sent to minister.
On the contrary, Dionysius says (Coel. Hier. viii) that the "Dominations are above all subjection." But to be sent implies subjection. Therefore the dominations are not sent to minister.
I answer that, As above stated (1), to be sent to external ministry properly belongs to an angel according as he acts by Divine command in respect of any corporeal creature; which is part of the execution of the Divine ministry. Now the angelic properties are manifested by their names, as Dionysius says (Coel. Hier. vii); and therefore the angels of those orders are sent to external ministry whose names signify some kind of administration. But the name "dominations" does not signify any such administration, but only disposition and command in administering. On the other hand, the names of the inferior orders imply administration, for the "Angels" and "Archangels" are so called from "announcing"; the "Virtues" and "Powers" are so called in respect of some act; and it is right that the "Prince," according to what Gregory says (Hom. xxxiv in Evang.), "be first among the workers." Hence it belongs to these five orders to be sent to external ministry; not to the four superior orders.
Reply to Objection 1. The Dominations are reckoned among the ministering angels, not as exercising but as disposing and commanding what is to be done by others; thus an architect does not put his hands to the production of his art, but only disposes and orders what others are to do.
Reply to Objection 2. A twofold reason may be given in assigning the number of the assisting and ministering angels. For Gregory says that those who minister are more numerous than those who assist; because he takes the words (Daniel 7:10) "thousands of thousands ministered to Him," not in a multiple but in a partitive sense, to mean "thousands out of thousands"; thus the number of those who minister is indefinite, and signifies excess; while the number of assistants is finite as in the words added, "and ten thousand times a hundred thousand assisted Him." This explanation rests on the opinion of the Platonists, who said that the nearer things are to the one first principle, the smaller they are in number; as the nearer a number is to unity, the lesser it is than multitude. This opinion is verified as regards the number of orders, as six administer and three assist.
Dionysius, however, (Coel. Hier. xiv) declares that the multitude of angels surpasses all the multitude of material things; so that, as the superior bodies exceed the inferior in magnitude to an immeasurable degree, so the superior incorporeal natures surpass all corporeal natures in multitude; because whatever is better is more intended and more multiplied by God. Hence, as the assistants are superior to the ministers there will be more assistants than ministers. In this way, the words "thousands of thousands" are taken by way of multiplication, to signify "a thousand times a thousand." And because ten times a hundred is a thousand, if it were said "ten times a hundred thousand" it would mean that there are as many assistants as ministers: but since it is written "ten thousand times a hundred thousand," we are given to understand that the assistants are much more numerous than the ministers. Nor is this said to signify that this is the precise number of angels, but rather that it is much greater, in that it exceeds all material multitude. This is signified by the multiplication together of all the greatest numbers, namely ten, a hundred, and a thousand, as Dionysius remarks in the same passage.
Source: Summa Theologica (New Advent)