You can modify the KeyColumns property of an attribute. For example, you might want to specify a composite key instead of a single key as the key for the attribute.
To modify the KeyColumns property of an attribute
In SQL Server Data Tools (SSDT), open the project in which you want to modify the KeyColumns property.
Open Dimension Designer by doing one of the following procedures:
In Solution Explorer, right-click the dimension in the Dimensions folder, and then either click Open or View Designer.
In Cube Designer, on the Cube Structure tab, expand the cube dimension in the Dimensions pane and click Edit <dimension>.
On the Dimension Structure tab, in the Attributes pane, click the attribute whose KeyColumns property you want to modify.
In the Properties window, click the value for the KeyColumns property.
Click the browse (...) button that appears in the value cell of the property box.
The Key Columns dialog box opens.
To remove an existing key column, in the Key Columns list, select the column, and then click the < button.
To add a key column, in the Available Columns list, select the column, and then click the > button.
If you define multiple key columns, the order in which those columns appear in the Key Columns list affects the display order. For example, a month attribute has two key columns: month and year. If the year column appears in the list before the month column, Analysis Services will sort by year and then by month. If the month column appears before the year column, Analysis Services will sort by month and then by year.
To change the order of key columns, select a column, and then click the Up or Down button.