You can have a list of Exclusions for product variants whose values you mostly want on certain products, with some exceptions. This is because a product might have many modifications/extras in common with other products, with only a couple of differences.
For example, you might want the Long Black to have identical coffee modifications to your other coffee drinks, with the exception of milk. This saves you the effort of creating a whole other “coffee modifications” product variant group specifically just for Long Blacks.
See that the “Long Black” (above) has identical Product Variant Values to the “Latte” (below), except for “Soy milk” and “Almond milk”.