Divrey Hayamim Alef 24:3

3 And Dovid with both Tzadok of the Bnei Eleazar, and Achimelech of the Bnei Itamar, divided them according to their pekudim (offices, assignments) in their avodah.

Divrey Hayamim Alef 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.

Divrey Hayamim Alef 24:3 In-Context

1 4 Now these are the divisions of the Bnei Aharon; the Bnei Aharon; Nadav, and Avihu, Eleazar, and Itamar.

2 But Nadav and Avihu died before avihem (their father), and had no banim; therefore Eleazar and Itamar became Kohanim.

3 And Dovid with both Tzadok of the Bnei Eleazar, and Achimelech of the Bnei Itamar, divided them according to their pekudim (offices, assignments) in their avodah.

4 And there were more Rashei HaGevarim found of the Bnei Eleazar than of the Bnei Itamar, and thus were they divided. Among the Bnei Eleazar there were sixteen Rashim l’Bais Avot, and eight among the Bnei Itamar according to the Bais of their Avot.

5 Thus were they divided by goralot (lots) with one another; for the Sarei Kodesh and Sarei HaElohim, were of the Bnei Eleazar, and of the Bnei Itamar.