Each App license can ONLY be activated for one ETS License
Example
You need one App for 3 different ETS6 Professional licenses. In this case, you have to purchase 3 App licenses and activate each one separately to its related ETS6 license. To activate your App follow these instructions: ETS6 Apps installation license activation
Conclusion
An App is always activated on one ETS License via its associated dongle. Once the App has been activated, you can NOT delete it or move it to another dongle. Therefore, think carefully before choosing the dongle you want the App to be activated on.
Exception
After upgrading (e.g. from an ETS4/5 to ETS6), all Apps assigned to the older version can be upgraded free of charge to the updated ETS6.
In this particular case please follow these instructions: App upgrade after an ETS upgrade