By selecting a product and navigating to the Inventory tab, you will be able to create substitutions.
If a customer orders the option selected on the left and it is unavailable, you can fulfill the order with the option and quantity on the right.
This is useful if you only pre-pack eighths but want to sell quarters, halves and ounces for lower prices per unit.
It also allows you to keep larger units on the menu, so a customer can still buy an eighth even if you only have gram bags ready.