See this article on how Microsoft is doing updates with W10.
Understanding the difference between Windows 10 feature and quality updates
" On Windows 10, there are four types of quality updates, including "B," "C," "D," and "out-of-band."
"The
"C" release happens the third week of the month, and the
"D" release outs during the fourth week of the month. Unlike the "B" rollout, the "C" and "D" updates contain a preview of the maintenance bits that the company plans to ship the following month during Patch Tuesday, and they don't include security updates. "
C & D fit the description of an "Optional Quality Update", a preview of non-security, maintenance updates.