A Modifier is an option that can be applied to an Item to customize it. For example, a customer purchasing coffee would have the option of adding whipped cream, or a pizza customer could add different toppings to the pizza.
Each Modifier has an associated cost which adds to the base cost of the Item.
Closely related Modifiers are grouped together to form a Modifier Set. You can then associate the Modifier Set with specific Items so that when the Items are added to a cart, an option will automatically be available to select from the applicable Modifiers.