Create Unbalanced Level Hierarchies Within a Dimension
You can use subdimensions to provide different levels of detail for specific categories, also known as unbalanced hierarchies.
For example, some branch offices may report product sales down to the item level, whereas others may report only to the product level. You can create a subdimension for those branches that report to the item level.
Similarly, your time dimension may contain levels for year, quarter, and month, but your OLAP report users may not need to see the month values for the previous year. You can use a subdimension to retain month levels in the current year while removing them from the previous year.
Categories in a subdimension are independent of levels in other parts of the dimension. Changes made to levels outside the subdimension do not affect categories in a subdimension, and vice versa.