When you want records to be updated, you can either use the Unique ID as a hidden form field and simply create custom links that pass the ID to your pages, or you can use it as a visible field on your forms. Let’s say you have a deal that needs to be updated with new information, and you want to ask your client to provide you with the information. You can send an email with a custom URL that passes the Unique ID to a hidden form field, so all your customer sees are the fields for the new information you need, and their responses will be stored in the right deal record.
This feature also boosts security when passing contact information through URLs because you can use the Unique ID instead of your contact’s email, which means that their private information will not be visible to anyone viewing the custom URL that sends your contacts to update their information.
The Unique ID field can be found in the system information section of all contact and custom object records.