brandpeak makes a difference between categories of product brands and trademarks.
These marks brand only one product or a smaller range of products. Examples are Ariel or Mars.
Range- or Family-Marks
A brand family stands for a whole range of products with a wider compentence. For example: Nivea. The brand should have a wider semantic and semiotic field; it should not be positioned to sharp.
A system mark brands a product under an umbrella brand. Known samples within the automotive area are Series 3 and Series 5 of BMW or the Class E and Class S of Mercedes Benz. They just structure the range but haven’t a brand power of their own. They are mostly used by stronger umbrella brands which do not need any separating power. (Products of Smart seem to be to weak to be Mercedes-Benz. They got a brand of their own.)
In dependancy of the existing brand architecure of the company product brands should be strong or weak.