Из BOL:

If a set of tuples is supplied as the slicer expression, MDX will attempt to evaluate the set, aggregating the result cells in every tuple along the set. In other words, MDX will attempt to use the Aggregate function on the set, aggregating each measure by its associated aggregation function. The following examples show a valid WHERE clause using a set of tuples:

WHERE { ([Time].[1st half], [Route].[nonground]), ([Time].[1st half], [Route].[ground]) }

If the «slicer_specification» cannot be resolved into a single tuple, an error will occur.

For more information about the Aggregate function, see Aggregate.