[Quoting from Facebook]

Supplementary Feeds supplement the product information provided by another data feed in the Commerce Manager. They can add or update certain fields for existing items but can’t create new items or delete items. 

Adding a supplementary feed in your catalog allows you to provide more attributes to your products. Consequently, it will help buyers to get more information about the product. Not only that, it allows you to provide attributes that are product specific, which in turn helps in enhancing the filter results.

Supplementary feeds only enhance the richness of the existing information, without altering any pre-existing information such as the Title, Content ID, Availability, Price, or Quantity. 

Under the “Products” section of the app, your products appear in two categories, namely: Primary Feeds and Supplementary Feeds.