In addition to the store products, you may as well sell in-store/restaurant prepared meals and this is when you'll be working with Product Modification Items. Here's a step-by-step guide for you to create a Product Modification Item.
1. Navigate to the Catalog on the left sidebar.
2. Expand the Product Modification Items menu and select the store where you want to add modification items.
3. Click the +Add Modification Item button.
4. Insert the basic information about your product.
Title: The name of your modification item.
Description: Short explanation of what the item represents and the ingredients it contains.
Store: To which of your stores this modification item belongs.
$: Put a price in USD, so that when the customer selects this modification item the total price of the product changes. You may put 0 as well if you don't want to charge your customers for this additive.
5. Hover over the image on the right and upload the main image of this item. Note that the image dimensions limit is 512px.
6. If the product is currently out of stock disable the Is in stock button, so the item is not visible to your customers upon the purchase.
7. Double-check if everything is correct and click Save.
Now that you have a new modification item, learn how to add it to a specific product.