Pass Thru fields are designed to pass a value from your checkout form to your servers, where you can either store or respond to the data. For example, if you have a checkbox for “Join Mailing List” on your checkout form and you pass this data through as a custom field, your servers can respond by adding the customer to your mailing list.
While Pass Thru custom fields are configured in the Control Panel, they can only be used to pass data via the API.
