When people submit responses to your form, they get a receipt number that is unique to them. To make it easy to tell which form a receipt number is linked to, give it a prefix.
To assign receipt prefixes to a form, open it from Forms, then go to Settings > General settings. In the Receipt number prefix field, enter whatever text you want. Remember to save your form when you’re done.
Now your responders will get form-specific receipt numbers. You also have control over the other information they see when they submit a form - check out Choose what form responders see after they submit to find out more.
If you're using payment gateways in your forms, it's best to assign each form a unique receipt number prefix. Some gateways can reject payments from multiple forms using the same prefix.