We're rapidly approaching the October 17 launch of the Windows 10 Fall Creators Update, and Microsoft is starting to wrap up testing with Insiders. In its latest move, Microsoft has started rolling out Windows 10 preview build 16299.15 to Insiders on the Slow ring.
Build 16299.15 first hit the Fast ring on October 2, and it doesn't contain any new features or notable fixes to speak of. What it does represent, however, is the Fall Creators Update's move to Release To Manufacturing (RTM) status, which marks 16299 as the build Microsoft will service with cumulative updates going forward.
Upon its release to the Fast ring, Microsoft indicated it would begin servicing this build with Cumulative Updates via Windows Update. We'd also expect that build 16299 will make its way to the Release Preview ring in one form or another ahead of the Fall Creators Update's release on October 17.