X-Plane Mobile Subscriptions
The latest version of X-Plane Mobile offers global airports and scenery for the entire world. This feature is available via a subscription.
If you prefer not want to subscribe, the free functionality of the app will not change. You will still be able to 1) use the aircraft that you have purchased and 2) fly around the legacy regions like LOWI, KSEA, etc.
The Professional Subscription offers global scenery including more than 36,000 airports, 11,500 of which with custom made 3D scenery. That includes features like terminals, hangars, etc. This subscription also gives you access to the entire fleet of aircraft for as long as you are a subscriber. With Professional you will also have early access to multiplayer.
The Basic Subscription is less expensive but only offers global scenery. You will be able to use only the free aircraft and those that you have purchased individually.
The best value for each of these is the annual subscription. This will save you roughly 30% in comparison to paying monthly.