Custom fields are the fields which are being displayed on order form to allow collect customers information when they purchase documents on your site via Documents Seller.
When you first install Documents Seller, the extension following custom fields (which is called core fields) : First Name, Last Name, Organization, Address, Address2, City, State, Zip, Country, Phone, Fax, Email, Comment
Go to Documents Seller -> Custom Fields to access to custom fields management page.
Since custom fields are being displayed on order form, you usually need to do the following tasks related to custom fields: