Custom fields are only applicable for buy now, shopping cart and subscription buttons.
Add a dropdown menu with price/option - if selected, you can define a dropdown menu that charges a different amount depending on what the user selects. You can also assign advanced discounts, advanced shipping profiles, or define a specific file to send to the user if they select the specific option. You also have the option of turning the dropdown options into radio options.
Add a dropdown menu - This is the same as the price/option menu with the exception that it does not associate prices with each option. If you need multiple dropdown menus without the need to associate advanced discounts or shipping fee profiles, use Add additional custom fields.
Add additional custom fields - You can define multiple extra fields using this option. Add dropdown list, text field, radio list, and/or a date picker.