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.
(..) Hey, it fucking involves lying and I'm pretty fucking good at that. Right? Right? Maybe because it's always been so easy... ...for me to get cunt that I never understood jerking off in a theater. Christ. Who knows what's easy? Colin, I know you'll take care of business. (..)2006 The Departed
(..) - And fire! State police, search warrant! State police, open the door! - Go, go, go! - Sir, yes, sir! - Is that understood? - Sir, yes, sir! Move it, get the fuck down right now. Get down, right now. Congratulations on passing the detective's exam, and welcome... (..)2006 The Departed