This was told by Dona Sarkar on Feedback hub. The full statement says:
To stabilize the Windows 10 Fall Creators Update for release, we have “forked” the release into its own branch we call “RS3_RELEASE”. Many Windows Insiders will recall we did the same thing for the Windows 10 Creators Update when we started flighting builds from the “RS2_RELEASE” branch. Going forward, Windows Insiders in the Fast ring will continue to get builds for the Windows 10 Fall Creators Update from this branch. However, Windows Insiders who choose to skip ahead will receive builds from the RS_PRERELEASE branch.To summarize:
- Fast ring: Builds from RS3_RELEASE branch with no inbox app updates.
- Fast ring + Skip Ahead: Builds from RS_PRERELEASE with inbox app updates from the Store.
Dona has also said how the things would go. After Fall Creators update is released to public, all of the fast ring Insiders would get build from same branch.
At some point in the future, build numbers in the RS_PRERELEASE branch will jump ahead, engineering teams will start checking in new code, and things diverge. When this happens – you will not be able to leave Skip Ahead and go back to builds from the RS3_RELEASE branch without reinstalling Windows. Once the Windows 10 Fall Creators Update is released to the general public, this skip ahead option will go away and the Fast ring will be back to receiving builds from the RS_PRERELEASE branch for the next Windows 10 release and inbox app updates from the Store will be fully restored to all Windows Insiders in the Fast and Slow rings.
This is how the settings would look like(Thanks to WinCentral)