1 Chronicles 24:3

3 And David parted them, that is, (with the help of) Zadok, of the sons of Eleazar, and Ahimelech, of the sons of Ithamar, by their whiles, and their service; (And David, with the help of Zadok, of the sons of Eleazar, and Ahimelech, of the sons of Ithamar, divided them for their duties, and their service;)

1 Chronicles 24:3 Meaning and Commentary

1 Chronicles 24:3

And David distributed them
The posterities of Eleazar and Ithamar into certain classes or courses:

both Zadok of the sons of Eleazar, and Ahimelech of the sons of
Ithamar;
these were the two principal men of the respective families, the one was high priest, and the other "sagan" or deputy; or they both officiated as high priests alternately, until the one was removed in Solomon's time, and the other established alone. And now their brethren the priests, who were under each of these, are the persons David divided into classes:

according to their offices in their service:
to perform their office as priests in the service of the temple in their turns.

1 Chronicles 24:3 In-Context

1 Forsooth to the sons of Aaron these portions shall be; the sons of Aaron were Nadab, and Abihu, Eleazar, and Ithamar; (And these shall be the divisions of the sons of Aaron; Aaron's sons were Nadab, and Abihu, Eleazar, and Ithamar;)

2 but Nadab and Abihu were dead without free children before their father, and Eleazar and Ithamar were set in priesthood. (and Nadab and Abihu died before their father did, and had no children, and so Eleazar and Ithamar became priests, that is, the High Priests.)

3 And David parted them, that is, (with the help of) Zadok, of the sons of Eleazar, and Ahimelech, of the sons of Ithamar, by their whiles, and their service; (And David, with the help of Zadok, of the sons of Eleazar, and Ahimelech, of the sons of Ithamar, divided them for their duties, and their service;)

4 and the sons of Eleazar were found many more in the men princes, than the sons of Ithamar. And David parted to them, that is, to the sons of Eleazar, sixteen princes by their families (and houses); and to the sons of Ithamar eight princes by their families and houses. (and there were found many more leaders of the sons of Eleazar, than of the sons of Ithamar. And David ordained to them, that is, to the sons of Eleazar, sixteen leaders by their families and houses; and to the sons of Ithamar eight leaders by their families and houses.)

5 And he parted ever either families among themselves by lots; for there were princes of the saintuary, and princes of the house of God, as well of the sons of Eleazar, and of the sons of Ithamar. (And he divided both families among themselves by lots; for there were officers of the sanctuary, and officers of the House of God, from the sons of Eleazar, and from the sons of Ithamar.)