Recently, we added a new feature to checkbox, radio button, and select list fields that allows you to make these fields unique. There are many practical uses for this.
For example, say you have 50 seats available for a charity dinner. You could make a checkbox field with all of the seat numbers on it, letting users purchase one or more seats (use a select list or radio button field if you want to limit them to one seat instead of letting them choose multiple seats). Each time the form is submitted, any seats selected will be removed from the list. This prevents the same seat from being reserved by two different users.
To use this feature, add a checkbox, radio button, or select list to your form as you normally would, using the Add Field button. Then click the Unique checkbox for that field. Now each selection made from that field when the user fills out the form will be removed. When no options are left on the form, if that field is required, then no one will able to submit to the form. If you go into your data and delete a submission, the options selected for that submission will be added back to your form.
This feature will be useful for those selling a set number of an item, taking a set number of reservations, etc. and wanting to close the form after all items have been selected.