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.
(..) Gotham gets blown to hell. The people of our greatest city are resilient. They have proven this before... and they will prove it again. one thing must be understood above all others. People of Gotham, we have not abandoned you. What does that mean? It means we're on our own. (..)2012 The Dark Knight Rises
(..) so that you can watch them clambering over each other to stay in the sun. You can watch me torture an entire city. And then, when you have truly understood the depth of your failure... we will fulfill Ra's Al Ghul's destiny. We will destroy Gotham. And then, when it is done... and Gotham is... (..)2012 The Dark Knight Rises