Albert Barnes Commentary 2 Chronicles 31:17

"and them that were reckoned by genealogy of the priests by their fathers` houses, and the Levites from twenty years old and upward, in their offices by their courses;" — 2 Chronicles 31:17 (ASV)

Both to the genealogy of the priests ... - Some prefer the translation: “And as for the list of the priests, it was according to the houses of their fathers, and that of the Levites was from twenty years and older.”

The author is explaining the nature of the lists that guided the officers who made the distributions. Three lists are described: first, a list of the priests, organized according to their fathers’ houses; second, a list of the Levites, including all who were twenty years of age and older, organized by their service divisions; and third, a list of the priestly and Levitical families mentioned in 2 Chronicles 31:18.