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.
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.
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.