Processing classes in SAP HR determine how wage types are processed during payroll runs. Custom processing classes can be created for specific requirements. To create a custom processing class: 1. Maintain the new processing class and its specifications in table V_T52D1. 2. Create a personnel calculation rule to define how wage types with the new class will be processed. 3. Edit the payroll schema to insert the new calculation rule. 4. Assign the new processing class to the relevant wage types in table V_512W_D.