I found it already (did not read properly, my bad):
The Pro version license is bound to the respective App Store account (e.g. on Windows: your Microsoft account). That allows you to use the same license on multiple devices of the same platform (e.g. Windows). Because different platforms are using different App Stores (e.g. Windows: Microsoft Store, Android: Google Play Store, …), you need to buy the Pro version separately for each platform you want to use it on.
