Every submission in Formstack is assigned a unique submission ID, which is unique to the entire Formstack system, not just a specific form or account. These numbers are never repeated, so you can use them for a variety of purposes. One thing our customers have used this ID for over the years is for an order or reservation confirmation number.
You can view the Submission ID for each submission by clicking on an individual submission on the Submissions tab of your form. The Submission-ID can be inserted into notification or confirmation emails, as well as well as custom messages that appear after the form is submitted.
To insert the Unique ID into an email, go to Settings > Emails & Actions > Confirmation Email, set the Contents radio button to 'Custom Message', and drag and drop the Submission ID variable from the Form Field Panel into the message. When the user submits the form, the variable for Submission-ID will be replaced by the actual number that Formstack generates.