This returns members that match a certain attribute value using the Filter function.
{FILTER( {TM1SUBSETALL( [Product] )}, [Product].[Category] = "Customer Lending")}

This example looks at multiple attribute values to return a filtered list:
{
FILTER(
    {TM1SUBSETALL( [Product] )},
    (
    ([Product].[Category]="Customer Lending" OR [Product].[Type]="Debit")
    AND
    ([Product].[Internal Deal]<>"No")
    )
    )
}

Filtering by level, attribute and pattern are combined in the following example:
{TM1FILTERBYPATTERN( {FILTER( TM1FILTERBYLEVEL({TM1SubsetAl([Product])},0),
[Product].[Internal Deal] = "Yes")}, "*ID??") }