When you install Revit 2023, the installer asks for both codes. The product key unlocks the specific Revit variant (e.g., Revit LT vs. Revit Building Design Suite). The serial number verifies that your license is genuine and activates the software with Autodesk’s servers.
If you purchase Revit 2023 from a reseller or distributor, they will provide you with a serial number and product key.
If you do not have a commercial serial number for Revit 2023, you have legal options: