The next feature update to Windows 10, codenamed Redstone 4 and likely called the Spring Creators Update, is right around the corner. The expected 1803 version number means that it will be finalized next month, and that means that Microsoft will be freezing app updates in the Fast ring. The company does this so Insiders can test the inbox apps that will ship with the update.
With the last version, Redstone 3, Insiders were able to get around this by opting into a Skip Ahead branch of the Fast ring, which eventually resulted in getting Redstone 4 builds before everyone else.
Now, Microsoft will be holding preview programs for individual apps, which will be completely independent of the Windows Insider Program. We've spotted it in a recent update to Photos, but Thurrott.com is reporting that it's also coming to other inbox apps like Camera, Feedback Hub, Alarm and Clock, and Tips.
During a development cycle, inbox apps - the apps that come pre-installed on Windows 10 - usually target the upcoming feature update, so if you opt into one of these programs, you still won't get the same preview app on a production build of Windows as you would on the Fast ring.
Similar to Skip Ahead, it's my understanding that space is limited for these individual app preview programs, so if you want it and it's available in the settings section of the app, you should sign up as soon as possible. The Windows Insider team will likely be announcing the feature in the coming week, so we'll learn more then.