This switch is located at Channel>Channel Settings>Ebay Settings tab.
When this switch is enabled, you can list your products in the secondary category. To correctly use this option, you need to obtain the additional category id from eBay and fill this in a custom field on your inventory database for all products you want to sell in the secondary category. If you don’t already have a custom field for this second category, you can create one.
IMPORTANT NOTE: Settings will be applied to the products with the second category id data in selected eBay channels.