Each response submitted to your form has a unique submission ID. This ID is unique to each submission and can be used in different ways. A good example is when Submission IDs are used when issuing order numbers to customers that have purchased an item from your online store. The submission ID can be sent as an order number after they have completed their order. Since the submission IDs are unique, you do not have to worry about repeating the order numbers.
Where can I find the Unique Submission ID?
The submission IDs for every response to a form can be found in is the first column of in the Responses sheet.
How To add submission ID in your confirmation email
Go to the Settings >> Autoresponder page of your form. See here to learn how to create an autoresponder message. Include the submission ID in the message sent to the respondent as a confirmation email just as you have inserted a form data. The submission ID tag has been included in the insert form data dropdown of the autoresponder modal as shown in the image below.
After you have added a confirmation email looks with the submission ID, the message appears in the email of your respondent as seen in the image below.