This feature actually resets any variant quantity to 0 if it is unmatched with feed file SKU and/or Barcodes.
When you check the first option, it will be enabled for all variants.
This option works best with the Filtering Shopify products.
If you have a blue dress (BD) with 5 quantity on your online store currently, and the feed file that you are trying to upload now does not have BD.
Stock Sync will reset the BD quantity to 0 in your online store.
If the feed file is detected empty, Stock Sync will not update anything.
Example 2 with Filtering Shopify products:
With the filter "Collection = test", "Published = Any" and "Vendor = X". Any product that unmatches those filters will reset the quantity to 0.
For the second option, it will only reset to 0 if it is unmatched against variants that are updated by Stock Sync before. "Unmatched only against variants that updated by Stock Sync before" is only applicable to products updated by the feed that enabled this. It will only start logging for updated products when this option is activated. Only products that updated by the specific feed and not found on the feed file will reset quantity to zero. Hope this help
If you have a blue dress (BD) with 5 quantity on your online store which was updated by Stock Sync previously, and the feed file that you are trying to upload now does not have BD.
Since the BD was updated using Stock Sync then Stock Sync will reset the BD quantity to 0 in your online store.