I...Butter is "a butter substitute" 
The references given (see discussion on this above) on this are improperly given. What is given appears to be little more than a google screen scrape. I managed to find both references and they do not support this usage. The use in these publications is not to define but to indicate the broad category. Peanut butter (if placed in the category of a 'nut butter') is also in the category if 'butter substitute' but it is not a type of margarine, which is what this product is. Possibly it might be helpful to say that the product is commonly used to substitute butter but it doesn't help in the basic definiton.