Quote:Windows 10 and updates, a never ending story. Michael Fortin, Corporate Vice President, Windows,revealed insights about the update testing and vetting process at Microsoft this week.

Microsoft releases cumulative updates on the second Tuesday of each month and refers to these updates as "B" releases. The company may also release updates in the third or fourth week of a given month, and refers to these as "C" or "D" releases.

These "C" and "D" updates are preview releases according to Fortin. Preview releases are releases that are still in testing; they will be delivered to devices on the next "B" release along with security updates automatically if automatic updates has not been disabled.

So-called "C" and "D" releases can be downloaded and installed by any Windows user by running a manual check for updates. In other words: if you select "check for updates", you may get pre-release updates that are none-security in nature, on stable versions of Windows.

Full reading: https://www.ghacks.net/2018/12/14/here-is-another-reason-to-avoid-check-for-updates-in-windows-10/