Preview features

Before functionality is fully released, also known as being generally available, it transitions through a number of different release tiers, such as experimental and preview. Preview features are those that are effectively available for app developers to use for the purposes of testing, in preparation for them being rolled out. 

The list of features that you see in this area changes from time to time, so it is a good idea to check them regularly and do regression testing on your apps to ensure that, when these preview features are enabled, your app still functions in the way that you expect.