According to MSPoweruser:
Along with the release of a Release Preview Build for Mobile, Microsoft has also released a new cumulative update for Windows 10 Version 1511 to Windows Insiders in the Release Preview Ring. The company has released build 10586.338 for PCs and it does not bring any new features as expected. However, this build probably comes with various improvements and bug fixes.
It is also worth noting that this build is not available to the general public, but if the build passes the testing process, it could soon be available to the rest of the users. So, if you are on the Release Preview Ring, you can download this update via Windows Update. – MSPoweruser