You must be wondering how to use the reset variants quantity to 0 if there is any SKU or Barcode unmatched in the feed file. 

Did you know that Filtering Shopify Products will affect this option? Because it makes the option much more precise and you won't get resetting everything to 0. 

How to avoid all being reset?

For example: 
You will notice it will only reset the variant quantity to 0 if it is unmatched against all variants under vendor Adibas. Basically, you need to include the vendor Adibas in the Filtering Shopify Products. So, it will only set the quantity to zero for the vendor Adibas only. 

You can click the Shopify Product Filter on the above image to set the filter you want to include. So that, it won't affect the rest of the products.