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.
(..) What's going on? Where are you going? You wouldn't have had anything to do with this, would you, Harold? # People just ain't no good # I think that's well understood There you go, boys. Just leave the bottle, Doris. Hey. Why the long face? It was all just a stupid mistake. (..)2004 Shrek 2