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.
(..) The joke's over. Let's just finish this. I've got a plane to catch. You're shittin' me, right? You know, I've never actually understood that expression, but, no, I'm not "shitting" you. Look, it's even got these idiot-proof tabs to make it easy. (..)2002 Sweet Home Alabama