Microsoft on Friday completed the development of Windows 8.1, the first major update to Windows 8 and Windows RT that will ship on new PCs and devices this coming holiday season. But this process—still called release to manufacturing (RTM)—isn't the end of the line for Windows 8.1. Instead, the update will continue to be improved and updated before its release to the public in October.

I exclusively revealed the RTM and build number of Windows 8.1—9600.16384.130821-1623—on Twitter on Friday, just as the Windows client team was signing off on it. I'm told that Windows 8.1 has already been provided to PC and hardware makers.

But the RTM version of this update, oddly enough, is not the end of the road for Windows 8.1 development. Instead, Microsoft will use the time between RTM and general availability (GA)—set for October 17, 2013—to continue testing Windows 8.1 in anticipation of releasing a set of interim fixes, called Quick Fix Engineering (QFE) updates. These QFEs will be delivered to PC and hardware makers so that they can add them to their PCs and devices before launch. End users will receive these QFEs automatically via Windows Update within three days of GA.

This process of "finalizing" a product and then updating it before it is released to the public isn't new; Microsoft did the same thing with Windows 8/Windows RT last year. But it does explain the two-month delta between RTM and GA, which many had questioned. RTM simply doesn't mean what it used to mean, and in this age of constant iteration, no product is ever really done.

Perhaps more surprisingly, Microsoft has made a somewhat startling admission, at least internally: In a congratulatory memo to his team, Jon DeVaan, the corporate vice president for Windows Development at Microsoft, noted that he expects Windows 8.1 to be much more reliable than Windows 8 and to have a "fighting chance" of approaching the reliability of Windows 7 "in some areas." This suggests that Windows 8 is known internally to be not as reliable as its predecessor, a fact Microsoft has never stated publicly.

As it turns out, the wide range of new built-in mobile apps like Mail, Calendar, People, Photos, and many others is what is holding back Windows 8's reliability. Some have argued to me in the past that these apps might be bundled with Windows now, but they're not "part of" Windows. But Microsoft's own internal metrics state otherwise: These apps—very much part of Windows—are what has held back the operating system's reliability scores. The new versions included in Windows 8.1 are expected to be more reliable.

Microsoft should publicly announce the RTM of Windows 8.1 sometime this week, sources tell me. Hopefully, the official announcement will include information about the firm's other plans for the release, including the availability of Windows 8.1 via MSDN and TechNet and any potential launch events.

The big question, of course, is what happens next. There are a few major milestones to consider.

First is a coming generation of new Surface 2 devices and third-party tablets running both Windows 8.1 and Windows RT 8.1. My sources tell me that Nokia is the only major hardware maker (aside from Microsoft) to embrace Windows RT, and its first tablet should be announced soon. But there will be many new Windows 8 devices, with various screen sizes, capabilities, and peripherals announced in the weeks ahead. Microsoft and its partners hope that this combination of new software and hardware will trigger renewed demand for Windows products, which have suffered a terrible year so far.

Second, Microsoft has vague plans to bring three major platforms—Windows client, Windows Phone, and Xbox One—more closely together. But it's unclear how or when this will happen. Windows Phone 8.1 is on an 18-month development schedule and won't ship until the first half of 2014. And with Xbox One rocketing toward its initial release amidst rocky competitive issues, it might be even longer before that platform can be updated to be more like the other two.

And what about Windows 8.2? Will Microsoft be able to continue this hectic pace going forward, or was Windows 8.1 the one-off service pack/feature pack mulligan that so many suspect? I don't have any answers there—my sources tell me Microsoft has no firm plans at all with regards to the future, in fact, given the recent reorg news and CEO Steve Ballmer's sudden exit—so the next year should at least be interesting.