In Admin > Preferences > Forms, you were able to create a custom form. One of the places you can use these forms is on the checkout page. It is a great way to capture additional information from the buyer that is related to the order itself.
Assigning a Checkout Form to your site
After the form you want has been created you'll need to assign it to the Checkout area. To do this go to Site > Settings > Scroll to the section labeled "Checkout Forms". The available forms list displays forms that you created under preferences. These forms may be assigned to the checkout page by selecting the form, and clicking the arrow to move them to the assigned forms list. The responses to the form will be shown in the order summary, in the buyers order history and in the admin order area and order reports.