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.
(..) He chose Jon to be his successor because he knew he had the courage to do what was right, even if it meant giving his life. Because Jeor Mormont and Jon Snow both understood that the real war isn't between a few squabbling houses. It's between the living and the dead. And make no mistake, my lady, (..)2016 TvShow: Game of Thrones Title: The Broken Man Season: 6 Episode 7