The license is based of your Apple ID through the app store. As long as you are logged into another iPhone using the same Apple ID for which you purchased the application the first time, you can place the SlingPlayer for iPhone up to ten iPhone devices.
But, if you bought the application on an iPhone and switch phones to an Android phone, you will be required to purchase the application again. That license for the SlingPlayer for Android phone can be placed on unlimited amount of Android phones, as long as you use the same Google Play credentials.
This would be the same as a Windows phone as well, have to purchase the SlingPlayer again.
If you do not know how to transfer credentials from one phone to another, I recommend contacting Apple (iPhone), Google Play (Android phones), or Windows (Windows Phones) to find out how to do so.
Hope this helps!!!
Sling Media Moderation Team