KMAT :
1)Suitable for MTO (Make to order)
2) Material configurable indicator checked in material master by default
3) Quantity and value update don't happen for this material type
4) According to the Customer specification you go for the product configuration.
FERT as configurable material:
1) Suitable for MTS (Make to stock)
2) You need to select the indicator "Material configurable" in basic 2 view in material master
3) Quantity and Value update happen for this material type
4) According to demand forecast you go for the product configuration