The serial number and product key for AutoCAD 2021 are used to activate the software and ensure it's genuine. Here's what I found:
It is important to note that for AutoCAD 2021, Autodesk transitioned significantly away from the traditional "Serial Number + Product Key" model toward a model.
While the serial number identifies your specific license or subscription, the product key tells the installer exactly which product is being activated. If you use a key for a different version (like 001L1 for AutoCAD 2020), the installation will fail even if your serial number is correct. Where to Find Your Serial Number
The you see (if the key is being rejected) Whether you are on a student, individual, or corporate plan