In the hierarchy schema element settings, you can specify a default member for a hierarchy. If you do not select the All Member check box, you must enter a proper MDX expression for the default member to avoid an MDX error when the end-user uses the hierarchy with a BI client application.
Calculated members, calculated measures, and named sets also use MDX formulas to specify how their calculations should be performed.
You can incrementally construct and test MDX formulas to validate the hierarchy default member using Microsoft Analysis Services 2000 MDX Sample.