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.
(..) "Will you please wake Georgy? We must go at once to the workers' city..." "No. , go back to your machine and forget this ever happened - understood?" "I must have someone who is faithful to me, Josaphat, how else can I fulfil my destiny?!" "I must go, Josaphat,- I must look (..)1927 Metropolis