Licensing Plan
Each Virtual DSM license pack adds one Virtual DSM instance to Virtual Machine Manager. A license supports Virtual DSM upgrades within a three-year period.

This license pack cannot be returned once opened.
Before purchasing this license pack, check the compatibility of your Synology NAS with Virtual Machine Manager. Learn more about applied models
This license pack can only be applied to one Synology product at a time.
This license pack can be used to extend the Virtual DSM upgrade support of an existing instance.

Activation and Mapping
In Virtual Machine Manager, go to License > Virtual DSM and click Add. Your Synology NAS must be connected to the Internet when you add the license.

Upgrade Period
Each Virtual DSM license comes with a 3-year software upgrade period for the Virtual DSM package. You can extend the upgrade period by activating a new license key for Virtual DSM.

More detail: https://www.synology.com/en-global/products/VDSM_License_Pack