What does Gao Qiao Petrochemical Company mean by Class Four and Class Two?

Shanghai Gao Qiao Petrochemical Company has a four-shift and two-shift system, that is, employees are divided into four shifts, two shifts every four days. One day shift and one night shift (each shift 12 hours) plus two days' rest, with four days as a cycle. This work arrangement is more common in pharmaceutical and chemical enterprises, thermal power plants, nuclear power plants and other enterprises.