Windows 10 update servicing cadence

I don't understand how "C" and "D" are optional updates - quoting Fortin:

https://blogs.windows.com/windowsexperience/2018/12/10/windows-monthly-security-and-quality-updates-overview/#y3E4GGsyWOCwxIsR.97

We also release optional updates in the third and fourthweeks of the month, respectively known as “C” and “D” releases.

... only to have

quality updates are cumulative and contain all previously released fixes to guard against fragmentation of the OS that can lead to reliability and vulnerability issues when only a subset of fixes are installed.  Most users are familiar with what is commonly referred to as “Patch Tuesday” or Update Tuesday.  These updates are published on the second Tuesday of each month, known as the “B” release (“B” refers to the second week in the month),

which is a forced update within a fortnight for 'Home' users....