Explanation/Reference:
A custom rollup technique, custom rollup formulas, lets the cube builder define an MDX formula for each dimension level. Analysis Services uses this formula to determine the value of the dimension level's members. For example, you could use an AVERAGE function rather than a summation to determine all members in one dimension level. If you use the AVERAGE function, the MDX formula for a dimension called Customers would be Avg( Customers.CurrentMember.Children ).
Note: The MultiDimensional eXpressions (MDX) language provides a specialized syntax for querying and manipulating the multidimensional data stored in OLAP cubes. While it is possible to translate some of these into traditional SQL, it would frequently require the synthesis of clumsy SQL expressions even for very simple MDX expressions. MDX has been embraced by a wide majority of OLAP vendors and has become the standard for OLAP systems.