What is the difference between the soft shaft and hard shaft in the linear motion shaft in use?

Linear motion shafts are divided into soft shafts and hard shafts. There are some differences in the specific use of these two shafts, mainly: the surface of the soft shaft is not hardened, so it cannot be used with linear bearings, while the surface of the hard shaft needs to be high-frequency quenched, and the hardness can reach about 60, so it can be used with linear bearings.