Nebulous writing is without a plan, a shape, a design. Just write. With the thought in the back of the head: think about where this piece, this novel, this screenplay is going.
Happens at some stage in a writing career. The fluid recording of thoughts, typed, or on paper, translated into something of a story, a piece, an article, a personal experience related, into a script of some important story.
In the end, nebulous writing isn’t nebulous at all, for all writing must go somewhere.