If an SSU is required then trying to install the CU without it will result in "not applicable" error message.
--please don't forget to
Accept as answer if the reply is helpful--
I can see that the servicing stack updates include a check for the version entry in "Component Based Servicing" registry key.
I also see that the Cumulative updates check for a UBR version within the registry.
Both of these versions appear to be similar in format and they are obviously from different registry locations but I suspect that they may be the method that is used to control when Cumulative Updates detect that the required SSU has been applied.
Can you confirm this?
Or is there a different set of guardrails in place to prevent installation of CUs before the required SSU has been installed.
NOTE: I am not referring to the combined SSU/CU bundles that I've heard about.