Product Bundle represent a collection of line-items sell together as a bundle, probably using discounted price. So we need to define when line-items a product bundle is refer to. This is the purpose of the line-items rules panel. In this panel you simply define one or more product variants. All the product variants together refer to as "Bundle". When some user purchase a bundle you will have  to provide the user with all the variants, specified by line-item rules. 

1. Click the "New" button

2. Select the product , variant and quantity.

3. Click save. 

This will add the variant to the line-items rules. Note that each line in the table refer to single variant type. If you need to change the quantity for example, you can click the edit button on the right side and edit the quantity for some specific variant.