Custom Field with Cart Attribute allows you to add some options to a product that can modify the product price. For example, you may wish to sell a book, or the downladable version of it. And the price is different in both case.
In this case you can select different variants. Default, Variant A,B, or C. The price will be modified by selection. In VM configuration you can also select either tax should be applied to cart variant in customfield.