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.
(..) Yeah, I thought so. Well, thanks to this little escapade of yours... ...I had to change shifts at work... ...so you have to pick up Aunt May tonight at . Understood? - Is that understood? - Yeah. Okay, good. She looks familiar. The girl on your computer. (..)2012 The Amazing Spider-Man