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.
(..) - No, never. Why do you ask? Mr. White? Mr. White? Yes? You understood what I've just said to you? Yes. Lung cancer. Inoperable. I'm sorry. I just need to make sure you fully understand. (..)2008 TvShow: Breaking Bad Title: Pilot Season: 1 Episode 1