There is a relationship between 데 (place), 는 데에 (situation), 는데 (connective) and 는데(요) (sentence-final). In particular, 는 데에 (situation) and 는데 (connective) are ambiguously similar with a continuous boundary between them. The best way to understand these grammatical forms is to understand how they ‘evolved’ from 데 meaning ‘place’ and see how they are related.