B class 20% relates to B class items are important. B class items has lesser important than A class parts still require more control than consumables (consumables = Paints, thinners, rubber products, radiator coolant, gasket sealants Ana bond etc .so apply FIFO on these items.) and will require some management effort.
C class parts are marginally important.
C classes make up the remaining. It is about 10%. C class requires the least controls, and will typically make up the largest volume of the stock.