We're now only 13 days away from the public release of the Windows 10 Fall Creators Update, which means that the Insider team is preparing a final release. It's widely believed at this point that build 16299.15 is the RTM, as it was released to the Fast ring a couple of days ago.
Now, the same cumulative update to build 16299 is being made available to Insiders that are using the Slow ring. It doesn't contain any major features, or even fixes; Microsoft says that it's testing the release of cumulative updates to the build, something that will have to be done at least monthly once it's generally available.
With the Fall Creators Update's release just around the corner, it won't be long now until some form of build 16299 hits the Release Preview ring, which is a ring that's reserved for builds that will eventually roll out to the public. You can sign up to receive the build now by going to Settings -> Update & security -> Windows Insider Program.