Numbers 4:2

Take the sum of the sons of Kohath from among the sons of Levi, after their families, by the house of their fathers,

The sons of Kohath were a family division within the sons of Levi, to be summed or counted out separately from among them for their appointed service. The counting was to be done by families and by ancestral households, with the Kohathites enrolled according to those divisions. This enrollment belonged to the organized assignment of duties for the tabernacle, under the orderly arrangements given to Israel through Moses.

Those counted for this service were men from thirty to fifty years old. They were appointed to carry the sanctuary's most holy things, including the sacred ark, only after Aaron and his sons had covered them. This transport was restricted to that particular family rather than left to any tribe or person who chose, as part of a larger system of disciplined, orderly movement and care for the tabernacle.