optional_numeric_expression is a numeric-valued expression that the function evaluates for each set element.
Typically, this expression has the form [MEASURES].[measure_name]
If you do not specify a numeric expression, the system uses the measure used by the current result cell. For example, this might be the measure used on the 0 axis or the measure specified in the WHERE clause, if any. If the query itself does not specify a measure, the system instead uses %COUNT, which counts records in the fact table.