Could You Use A Dutch Oven as A Casserole
Possibly a matter of the difference in depth between the two items? 

There's a Hungarian layered dish I make with chicken, potatoes, peppers and fresh tomatoes with sauce poured over before baking. If I were to make that in a more shallow casserole dish, I wouldn't even be able to do one layer. A Dutch oven is a necessity. But a lasagne made in a deep Dutch oven would end up swimming in liquid. I know 'cause I tried it once!

Other than examples like that, I'd think many foods could be cooked in either.