You can use custom labels in a product data feed to:
Tag products you’d like to group in a Shopping campaign by values of your choosing
Provide additional description information for shoppers in search results
Add supplemental image URLs
Submit product brand descriptions
With Shopping campaigns, you can use custom labels when you want to subdivide the products in your campaign using values of your choosing. For example, you can use custom labels to indicate that products are seasonal, on clearance, best sellers, etc. These values can then be selected to use for monitoring, reporting, and bidding in your Shopping campaign.
Read more here: https://support.google.com/adwords/answer/6275295?hl=en