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.
(..) An idea. Resilient. Highly contagious. Once an idea has taken hold of the brain, it's almost impossible to eradicate. An idea that is fully formed, fully understood, that sticks. Right in there somewhere. For someone like you to steal? Yes. In the dream state, your conscious defences are lowered... (..)2010 Inception