It is Rockwell hardness, HB is Brinell hardness and HV is Vickers hardness.

Generally speaking, HRC is widely used. For example, the hardness requirement after quenching and tempering can be expressed by HRC. HB is often used to indicate the hardness of base materials and castings, or the hardness is lower after annealing, normalizing and quenching and tempering. HV is usually used for microhardness, such as surface hardness after carburizing quenching or nitriding quenching. The hardness layer of these surface heat treatments is very thin, so HV is often used to express and measure its hardness value.