All that they do is to be under the supervision of Aaron and his sons—all their work and carrying assignments. You are to tell them all they are required to carry.
This was the total of the families of Kohath—all who were entitled to do the work of serving in the Tent of Meeting. Moses and Aaron registered them in accordance with the Lord's instructions to Moses.
This was the total of the families of Gershon —all who were entitled to do the work of serving in the Tent of Meeting. They were registered by Moses and Aaron in accordance with the Lord's instructions.
It was in response to the Lord's instructions that they were registered by Moses. Each of those registered were told what to do and what to carry, as the Lord had ordered Moses.
The Levitical priests must be there too, because the Lord your God has chosen them to serve him and to give blessings in his name, and to decide on legal disputes and cases of assault.