Charles Ellicott Commentary Numbers 4:31

"And this is the charge of their burden, according to all their service in the tent of meeting: the boards of the tabernacle, and the bars thereof, and the pillars thereof, and the sockets thereof," — Numbers 4:31 (ASV)

The boards of the tabernacle, and its bars ... — The duty of the Merarites was to carry the heavier and more cumbersome parts of the Tabernacle and the court; and it is probably on this account that, in their case, the specific duties of each were assigned to them by name (Numbers 4:32). The description of the several portions of the Tabernacle and Court here enumerated is found in Exodus 26:36.