4.5 Rollup Members for Dimensions Supporting Hierarchies
For dimensions that support hierarchies, every dimension member value has the property of being either a leaf value (child-only or last descendent child value) or a rollup node value. For example, if Branch #1, Branch #2, and Branch #3 all belonged to the West rollup point of an organizational hierarchy, then West would also be a dimension member value of the Organizational Unit dimension. Your business data is typically expressed at the leaf level.