When people say “they're engaged,” the phrase “to be married” 'is understood'. [=“they're engaged” is understood to mean “they're engaged to be married”]
We still don't fully 'understand' the causes of the disease. = The causes of the disease are still not fully 'understood'.
He believed it 'was understood' [=believed that everyone had agreed] that he would be paid for the work.
(..) - Harry tells me you're quite the science whiz. Y'know, I'm something of a scientist myself. I read all your research on nanotechnology. Really Brilliant. - You understood it? - Yes, I wrote a paper on it. Impressive. Your parents must be very proud. - I live with my aunt and uncle, they are proud. (..)2002 Spider-Man