Albert Barnes Commentary 1 Chronicles 24:19

"This was the ordering of them in their service, to come into the house of Jehovah according to the ordinance [given] unto them by Aaron their father, as Jehovah, the God of Israel, had commanded him." — 1 Chronicles 24:19 (ASV)

These were the orderings — that is, this was the established numerical order for their ministerial service in the house of the Lord. This service was to follow the method determined for them by their forefather Aaron, according to the instructions he received from God.