Synology licenses are divided into two types: lifetime licenses and subscription licenses( type1 and type2 ).
Lifetime licenses: Surveillance camera/device and MailPlus licenses. They are purchased once and can be used and upgraded permanently. They can also be migrated/transferred to another Synology NAS or Synology account.
Subscription license type1: Virtual DSM. Every 3 years will need to renew it for DSM upgrading. If not renewed, the existed Virtual DSM could also work but have no future DSM update.
Subscription license type2: Virtual Machine Manager Pro. Need to be renewed periodically to continue using.