Objection 1. It would seem that duties do not differ according to their actions. For there are infinite varieties of human acts both in spirituals and in temporals. Now there can be no certain distinction among things that are infinite in number. Therefore human duties cannot be differentiated according to a difference of acts.
Objection 2. Further, the active and the contemplative life differ according to their acts, as stated above (Question 179, Article 1). But the distinction of duties seems to be other than the distinction of lives. Therefore duties do not differ according to their acts.
Objection 3. Further, even ecclesiastical orders, states, and grades seemingly differ according to their acts. If, then, duties differ according to their acts it would seem that duties, grades, and states differ in the same way. Yet this is not true, since they are divided into their respective parts in different ways. Therefore duties do not differ according to their acts.
On the contrary, Isidore says (Etym. vi, 19) that "officium [duty] takes its name from 'efficere' [to effect], as though it were instead of 'efficium,' by the change of one letter for the sake of the sound." But effecting pertains to action. Therefore duties differ according to their acts.
I answer that, As stated above (Article 2), difference among the members of the Church is directed to three things: perfection, action, and beauty; and according to these three we may distinguish a threefold distinction among the faithful. One, with regard to perfection, and thus we have the difference of states, in reference to which some persons are more perfect than others. Another distinction regards action and this is the distinction of duties: for persons are said to have various duties when they are appointed to various actions. A third distinction regards the order of ecclesiastical beauty: and thus we distinguish various grades according as in the same state or duty one person is above another. Hence according to a variant text [the Septuagint] it is written (Psalm 47:4): "In her grades shall God be known."
Reply to Objection 1. The material diversity of human acts is infinite. It is not thus that duties differ, but by their formal diversity which results from diverse species of acts, and in this way human acts are not infinite.
Reply to Objection 2. Life is predicated of a thing absolutely: wherefore diversity of acts which are becoming to man considered in himself. But efficiency, whence we have the word "office" (as stated above), denotes action tending to something else according to Metaph. ix, text. 16 [Ed. Did. viii, 8. Hence offices differ properly in respect of acts that are referred to other persons; thus a teacher is said to have an office, and so is a judge, and so forth. Wherefore Isidore says (Etym. vi, 19) that "to have an office is to be officious," i.e. harmful "to no one, but to be useful to all."
Reply to Objection 3. Differences of state, offices and grades are taken from different things, as stated above (1, ad 3). Yet these three things may concur in the same subject: thus when a person is appointed to a higher action, he attains thereby both office and grade, and sometimes, besides this, a state of perfection, on account of the sublimity of the act, as in the case of a bishop. The ecclesiastical orders are particularly distinct according to divine offices. For Isidore says (Etym. v.): "There are various kinds of offices; but the foremost is that which relates to sacred and Divine things."
Source: Summa Theologica (New Advent)