Windows 10 update servicing cadence

You are correct, with Windows 10, all releases, Quality and Feature are cumulative, so subsequent releases are built on and contain all previous releases. 

To minimize end-user reboot disruption, most of our self managed customers, and as default for the devices we manage, configure to update once a month, to get the lastest security patches. These are the B releases.

We refer to the "C" and "D" releases as "optional" because"

• They only include quality fixes, not security fixes and therefore don't have the zero day exposure impications.
• The fixes will  come to you in the next "B" release, which is what we and most of you are focused on getting quickly installed when they come out. So unless there is a specfic fix that your blocked on, and thus need quickly, you will  get the improvements with the next "B", along with the new security fixes, and then only have on reboot. 

"C" and "D" are there to so that  if you want, you can deploy them early with your first flighting rings and have data and visibility to the changes before deploying the "B" release.

The fixes themselves are not optional, as you correctly called out, we only have cumulative updates now, but the specific update package that you deploy to get a set of fixes, and when is optional.