Known knowns - either a simple or complicated case. If simple, we do it routinely. E.g. landing an aircraft in good weather.
Known unknowns - we have a plan for accommodating it. E.g. landing an aircraft in Edmonton with high crosswinds.
Unknown unknowns - we are in a complex domain and we have to see how things should work with experiments e.g. edge of envelope flying. One of the things that the Kennedys made clearer for me is how experiments have to be well conceived, in particular, by controlling variables properly (see their SysEng Journal paper).
Unknown knowns - we didn’t realize we could plan for this but now that we sense it, we can delegate to existing routines, or perhaps adapt to it. That pathway is available but unused. The example would be .. looking through the manual and realizing the system can actually do this (maybe the Apollo 18 case? There they did the exaptation that Snowden often mentions.)
Snowden would no doubt criticize my limited understanding, but there is some use in seeing how these frameworks co-exist, for me at least.
Update: Dave Snowden replies with a pointer to his HBR article with Cynthia Kurtz in which “knowns” are discussed. Summary: simple _contexts are the domain of known knowns, _complicated contexts are the domain of known unknowns, since experts are required. Unknown unknowns are the domain of complex contexts. Interestingly, they categorize the Apollo 13 case as being in the complex context. In the sense that there was no clear answer, that makes sense, but to me, it also highlighted that idea of unknown knowns: that is, these skilled engineers did “know” the answer (since the astronauts survive), but not consciously. So we could perhaps characterize that as relying on the “expert within”.