A US Postal Service Delivery Confirmation Number is not the same thing as a Tracking Number, even though the post office often uses the words "Track and Confirm."
A Tracking Number will tell you exactly where a letter or package is located at that moment.
A Delivery Confirmation Number will tell you when and where that letter or package was delivered.
For example, when we send out a USPS/Priority Mail letter or package, we include a Delivery Confirmation Number at no charge in the shipment confirmation email that we send out. This has been very handy because if you have the shipment going to c/o General Delivery or someplace else for example, you can check online at www.USPS.com to see if the package has been delivered before you make the trip to pick it up, only to find that it isn't there yet.
It also comes in handy if you've checked and confirmed that it was delivered and you get to the post office and they tell you that they don't have it, you can speak with certainty and tell them that according to their web site, they received the package on such-and-such date and exactly what time was it that they scanned the shipment.