Microsoft appears to be beginning a new app preview programme. One that is separate from the existing Windows Insider test structure. Few details are being shared at the moment, but sources claim that invites to test several upcoming apps have already been sent out.
The previewed apps are being sent out under the Windows App Previews banner, and appear to be deployed in a manner similar to how Google tests its own apps. Thus far, it looks like previews for updated versions of Windows Camera, Microsoft Photos, Alarm & Clock, Feedback Hub, and Tips have been seen in the wild.
Microsoft, on its part, has said nothing about the existence of the such a preview programme. It would appear that this is something the company wants to try out, but is avoiding too much fanfare before it works out how to present the situation. No mentions of Windows Insider could mean that the company wants to separate the two. Perhaps to allow people to test the apps in a more stable environment to make it easier to catch potential problems.