Create table with cross join filtering

alexa_0028  does this improve

Calculated Table =
DISTINCT (
    SELECTCOLUMNS (
        ADDCOLUMNS (
            'Product',
            "L6",
                VAR _lookup = 'Product'[P. Code]
                VAR _looupValue =
                    CALCULATE ( MAX ( 'Mapping'[L2] ), TREATAS ( { _lookup }, 'Mapping'[P. Code] ) )
                RETURN
                    IF ( _looupValue = BLANK (), "not mapped", _looupValue )
        ),
        "Product Code", [P. Code],
        "Product Description", [Product],
        "L6", [L6]
    )
)

alexa_0028  is this any better

Calculated Table =
ADDCOLUMNS (
    SUMMARIZE ( 'Product', "Product Code", [P. Code], "Product Code", [P. Code] ),
    "L6",
        VAR _lookup = 'Product'[P. Code]
        VAR _looupValue =
            CALCULATE ( MAX ( 'Mapping'[L2] ), TREATAS ( { _lookup }, 'Mapping'[P. Code] ) )
        RETURN
            IF ( _looupValue = BLANK (), "not mapped", _looupValue )
)

Your SUMMARIZE looks strange to me. Did you mean to write this?

Calculated Table =
SELECTCOLUMNS (
    SUMMARIZE ( 'Product', 'Product'[P. Code], 'Product'[Product] ),
    "Product Code", 'Product'[P. Code],
    "Product Description", 'Product'[Product],
    "L6",
        VAR _lookup = [...]
)