In this panel you will get custom field list. There is an add new button for new field and edit button for edit old field. the add/edit button is look like the image below:
In this add/edit panel, you can enter the label, help text, type, is required,etc. You can choose the field for specific category or all category. You can set the field for API validation envato. If you want to check purchase key of envato then you have to select from Choose API Dropdown . if you want to check that on submission then you have to enable this "Check on Submit" .