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.
(..) believe it or not. Oh, I believe it. Mummies never die, so they tell me. Sarah, my daughter... Galaxies, the universe, whatnot... She always understood all that stuff. I always figured there was enough going on down here, I never needed to look elsewhere. Your daughter, what grade is she? (..)2016 TvShow: Stranger Things Title: Chapter One: The Vanishing of Will Byers Season: 1 Episode 1