CBP is the past consumption values of stock; it is used to forecast future requirements. On the basis of past consumption values, the net requirement of goods is calculated.
The difference between CBP and MRP is that when you plan materials using MRP, you have to predict the Materials Requirement(MR) based on sales and operations planning (SOP). While in CBP you have to predict the Material Requirement(MR) based on historical demand for materials.
