Microsoft has released a new Windows 10 preview build, 10586, to Windows Insiders in the Fast Ring.

According to sources, this build is likely to be the one designated as the release to manufacturing (RTM) build of "Threshold 2" -- the Windows 10 release likely to be known officially as "Windows 10 Fall Update."

Yes, I know Microsoft officials are trying to stamp out use of "RTM," and are calling Windows 10 a regularly updated "service." But whether you prefer RTM or RTW (release to Web) or GA (generally available), this build is the one likely to get that official nod, sources say.

The new preview build for PC testers is almost entirely focused on bug fixes and general improvements, according to a November 5 blog post announcing the test build's release.

Among the fixes in 10586 are fixes to audio playing; Tab previews in Edge; and the random blue screens some small form-factor devices, like Dell Venue 8 Pro, were experiencing.

There are some known issues, including the possible disappearance of the Insider Hub and possible disappearance of Skype messages and contacts. There are workarounds listed in the blog post for both of these issues.

Windows 10 Fall Update is expected to lump together a number of the new features that Windows Insider testers have been evaluating since the initial July release of Windows 10. The Fall Update, which Microsoft is expected to start rolling out next week, according to various sources, will be delivered as a regular Cumulative Update, according to previously leaked information.

Among the new features expected in the Fall Update are improvements to Cortana and search; Favorites, Reading List and password syncing in the Edge browser (along with the ability to launch a second instance of Edge); and support for the recent Activation changes Microsoft made to allow Windows 7 and Windows 8.x product keys to work.

Enterprise Data Protection and support for the Windows 10 Store for Business -- which Microsoft made available to Insider testers as of November 4 -- are also both likely to be made available on or around the time of the Fall Update release.

What's not going to be part of Threshold 2 is support for extensions in Microsoft's Edge browser. That feature has slipped and is now slated to be part of "Redstone 1," the next major update to Windows 10, which will be out by June 2016, my sources have said. My sources are also saying that Windows Insiders will likely get to test Edge extensions as part of a Windows 10 Insider build before this calendar year is over.

I'm hearing Microsoft also may designate Windows 10 Mobile build 10586 as the "RTM" for its Windows 10 Mobile release some time in the not-too-distant future. Microsoft officials have said existing Windows Phone users will start seeing Windows 10 Mobile roll out to their phones in December 2015.