About T-code MI31

Dear experts,

Do you know how MI31 get the data? Where does the material data in MI31(which not create PI doc) come from?

As I understand, the materials are from MCHA and MCHB, seems something related to MBEW, but really not quite sure whether I am correct, because we want to rewrite the program and add some logic, but Not quite sure what table the program checked. So does anyone who can help me?