'Duty' in the Bible
"Look! The king's son is going to rule, just as the LORD promised David's descendants. So here's what you'll need to do: One third of you priests and descendants of Levi who are on duty during the Sabbath will serve as guards at the temple gates.
Nobody is to enter the LORD's Temple except for the priests and descendants of Levi who are on duty. They may enter because they are ceremonially holy, but all the rest of the people must observe the LORD's instructions.
What Jehoiada the priest ordered is precisely what the descendants of Levi and all of Judah did. Each of them took the men who were on duty on the Sabbath as well as those who were off duty. Jehoiada the priest did not release the divisions from service,
Amaziah gathered Judah together and organized them according to their ancestral households under commanders of thousands and hundreds throughout Judah and Benjamin. He then mustered an army from those who were 20 years old and older. He discovered that there were 300,000 elite soldiers qualified for war duty and capable of handling spears and shields.
without regard to genealogical enrollment, to every male 30 years old and older that is, to everyone who entered the LORD's Temple as their duty obligations required for their work and duties according to their divisions