Internal only.
This was setup by Will for Automotive Superstore.
This FAQ in internal but steps can be shared with the sellers.
This setting is on the Settings - Magento - General page.
Scenario:
The seller has over 300K products in Magento but only wants to sell some on the marketplaces.
Have the seller create an attribute and share it with you, then add it into the above field.
See example below.
https://m2.omnivore.com.au/configuration/feedConfig/automotive-superstore?feedType=magento2
In this case ebay was the attribute name and 1 means yes.
So if the product has that attribute set to 1 it will be imported using the filter.