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.
(..) ...my friend Mathis. You've taken good care of your body. Such a waste. You know... ...I never understood all these elaborate tortures. It's the simplest thing... ...to cause more pain than a man can possibly endure. And of course... (..)2006 Casino Royale