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.
(..) - I'm Christian Thompson. That's my way. - That's your way. Right. I work freelance. I have a lot of free time on my hands. You know, I never understood... why everyone was so crazy about Paris... but... it is so beautiful. Mm. I-I can't. I'm sorry. I can't. You know, Nate and I just split up (..)2006 The Devil Wears Prada