2. If you are looking to make all your products untaxable, then you can use the Update Product feed and input in the indicator for untaxable.
You can input any of the value indicators from your feed file for the untaxable. In this example, used FALSE as an indicator of untaxable.
4. If your feed file doesn't have any column for taxable and you want to apply the same value to all the products in the feed.
You can do this by enabling the toggle and entering the Static Value in the field instead of entering the column index or name. So, all the products with TRUE value will become taxable.