A U-bolt is a bolt in the shape of the letter U with screw threads on both ends. Carbon steel U bolt clearly means the material of the U bolt is carbon steel.

Carbon steel U bolt has primarily been used to support pipework, pipes through which fluids and gasses pass. As such, carbon steel U-bolts were measured using pipe-work engineering speak. Carbon steel U-bolt would be described by the size of pipe it was supporting. Carbon steel U-bolt are also used to hold ropes together.

For example, a 40 Nominal Bore U-bolt would be asked for by pipe work engineers, and only they would know what that meant. In reality, the 40 nominal bore part bears little resemblance to the size and dimensions of the U-bolt.