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.
(..) Three attacks on Muggle-borns. Things have gone far enough. -The Ministry's got to act. -But l never-- You know l never, professor. l want it understood, Cornelius. . . . . .that Hagrid. . . . . .has my full confidence. Albus, look, Hagrid's record is against him. -l've got to take him. (..)2002 Harry Potter and the Chamber of Secrets