We consider a situation in which a hierarchical planning system with two levels is applied to a multistage production process. The higher level aggregate production plan, expressed in terms of product groups, is disaggregated at the lower level into a detailed plan for individual items. We describe constraints imposed upon the aggregate level product groups that will guarantee the existence of a feasible disaggregation at the lower detailed level. The possibility of formulating suitable constraints is closely related to our choice of product groups and the aggregation of product data.