SAP Message W3120 - Default indicator for & & & cannot be reset

You have attempted to mark a default schedule line category as apossible schedule line category for the combination of item category ofitem category and MRP type. System Response The system does not accept this selection as this schedule linecategory is the only schedule line category which has been assigned tothe given combination of key fields. Procedure You must assign an additional schedule line category to the key fieldsgiven above. You can then mark this schedule line category as adefault.The system automatically carries out the change to the defaultID which it previously rejected.