Explanation/Reference:
Content Tab
Aggregation Content
You can define the aggregation (ie the group by) of:
by hierarchy level
by logical column
It helps you to define an aggregate navigation

Note: logical table source
Logical table sources define the mappings from a single logical table to one or more physical tables. A logicaltable contains one or more logical table source. The mapping between physical columns and logical columns are done in this element.
The definition of the logical table source is used:
to specify transformations/calculations (in the column mapping tab),
to enable aggregate navigation and fragmentation (in the content tab)
The Content tab of the Logical Table Source is used to define :
any aggregate table content definitions (specify the grain of the physical tables), fragmented table definitions for the source,
and WHERE clauses (if you want to limit the number of rows returned).