Once you buy the Full Version of iStudiez Pro on a certain platform, you may use it on any of your devices of that OS.
For example, you've bought the Full Version for iOS.
iPhone and iPad both run on iOS, so you don't have to buy the app for iPhone and again for iPad.
The Full Version of the app should be bough on every platform where you want to sync the app's data.
So, if you have bought the Full Version for iPhone and want to sync on your Windows PC, you should buy the Full Version for Windows too.