--  ISSUBTOTAL checks if the current row in SUMMARIZE is 
--  a subtotal for a column used in a ROLLUP
--
--  It is superseded by SUMMARIZECOLUMNS that has the 
--  option of ROLLUPADDISSUBTOTAL, combining both ROLLUP
--  and ISSUBTOTAL in the same function.
EVALUATE
CALCULATETABLE (
    SUMMARIZE (
        'Product',
        ROLLUP ( 'Product'[Brand] ),
        "IsSubtotal", ISSUBTOTAL ( 'Product'[Brand] ),
        "Sales", [Sales Amount]
    ),
    Product[Brand] IN { "Contoso", "A. Datum", "Proseware" }
)