Measures and Measure Groups - Expert Cube Development with SSAS Multidimensional Models - page 83

If you don't associate a calculated measure with a measure group, but place it in
a folder, the folder will appear at the same level as the folders created for each
measure group.
Built-in measure aggregation types
The most important property of a measure is AggregateFunction ; it controls how
the measure aggregates up through each hierarchy in the cube. When you run an
MDX query, you can think of it as being similar to a SQL SELECT statement with
a GROUP BY clause; whereas in SQL, you have to specify an aggregate function to
control how each column's values get aggregated, and in MDX you specify this for
each measure when the cube is designed.