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.
(..) I need a nightlock pill, so I can die when I need to. If it gets to that point, I'll kill you myself. I don't think I ever really understood the capitol until now. You eat like this, you'll believe anything. So Katniss Everdeen, a poor unstable girl (..)2015 The Hunger Games: Mockingjay - Part 2