BigCommerce tax classes are used on listings to determine what tax rate should be used for that particular listing when it's ordered.
These are set up directly on BigCommerce and then we import them into StoreFeeder so that you can add them to your listings. If you've recently added a new tax class that you need to use on some listings if you go into your BigCommerce integration and go to the tax class tab on the left there should be an "Import Tax Classes" button on the right:
Once you've imported them you should find that your newly imported tax classes should be available to use when you edit your listings:
If you need to edit the tax class of a lot of listings at once we recommend using the listing import export, you can find a knowledgebase of how to use that here.