Microsoft began rolling out the Windows 10 Mobile version of its Creators Update release at the end of the day Pacific Time on April 25.

The RTM ("release to mainstream," if you're not a fan of "release to manufacturing") version of the OS for Windows Phones is 15063.251. (The RTM for Windows 10 Creators Update for PCs also was 15063.)

As Microsoft acknowledged recently, only about a dozen existing Windows Phones will be getting the RTM of the Creators Update. Windows Phones that are not on the short list will be able to stay on the Creators Update preview even though it's unsupported, Microsoft officials said. Unsupported phones won't be able to get future preview builds beyond the Creators Update release, however, officials said.

The Windows Phones eligible to get Windows 10 Mobile Creators Update:

Alcatel IDOL 4S

Alcatel OneTouch Fierce XL

HP Elite x3

Lenovo Softbank 503LV

MCJ Madosma Q601

Microsoft Lumia 550

Microsoft Lumia 640/640XL

Microsoft Lumia 650

Microsoft Lumia 950/950 XL

Trinity NuAns Neo

VAIO VPB051

Microsoft has been releasing early preview builds of the next feature update for Windows 10, codenamed "Redstone 3," to PC testers in the Fast Ring. The company has also released something called "Feature 2" (builds in the 15200 series) to Insiders running the handful of Windows Phones that are eligible to get the Creators Update RTM.

Feature 2 seems to be a splinter branch of Windows 10 that isn't the same as Redstone 3. I asked Microsoft if officials would clarify what's going to happen with Windows 10 Mobile OS moving forward and a spokesperson said the company had nothing more to share beyond its blog post noting that Windows 10 Mobile test builds wouldn't have the same numbering as the PC versions.

See also: Windows 10 Creators Update: The smart person's guide (TechRepublic)

My -- and other Microsoft watchers' guess -- is that Microsoft plans to make its upcoming Windows 10 on ARM version of the operating system its future mobile OS platform. Windows 10 Mobile will likely continue to be supported for some period of time, as Microsoft is continuing to market it to specific vertical markets.

Microsoft officials said yesterday, April 25, that "millions" of Windows 10 PCs already are running the RTM version of the Creators Update, two weeks after it became generally available.

As company officials said previously, Microsoft is taking a phased approach to the Creators Update rollout and repeated again that only tech-savvy users attempt to proactively download the final Creators Update bits. Microsoft's guidance is to wait for the company to push Creators Update to devices via Windows Update to minimize possible compatibility and other issues.

The Windows 10 Mobile Creators Update release includes only a subset of the features that are in the PC version. A list of some of the new more business-focused features in the Mobile release -- including SD card encryption, remote PIN resets for Azure Active Directory accounts, WiFi Direct Management, and a few more -- are in this Microsoft article on IT pro content in version 1703, aka the Creators Update. (Thanks to @WinObs for the link.)

Windows 10 offers new Mail and Calendar features to non-Microsoft account users: