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.
(..) Just by knowing it. How else? But I believe in reason. Reason. Was it reason that made you choose your wife? I came looking for you. I understood something. What did you understand? He smiled at me. He's very advanced for his age. What did you understand? I'm so proud of you. (..)2012 Anna Karenina