User hierarchies are sets of attributes and their relationships, arranged in specific sequences for a logical business organization. You create user hierarchies to define the browse and drill relationships between attributes. For example, you can create a Time hierarchy that contains the Year, Quarter, Month, and Day attributes. When you browse the attributes in the Data Explorer, you can double-click Year to get to Quarter and double-click Quarter to get to Month, and so on.
Whereas browsing occurs through the Data Explorer, in drilling the user actually chooses to move to higher or lower levels on a report or move across to levels within different hierarchies. For example, if the user drills on the Quarter attribute in a report, he or she can drill down to Month, up to Year, or across to an attribute within another hierarchy.
A user hierarchy is the only type of hierarchy you can define, and you can create any number of user hierarchies for each project. You should define user hierarchies that correspond to specific areas of your company business model and data warehouse schema.