The Substantial Projects board is for RM projects that have significant work done on them, such as mapping, a working engine, and playable content. Projects of all RMs can be found here, as long as they are judged to be on their way to completion by a Moderator or Administrator.
Although "substantial content" isn't well defined in the board itself, a look at the projects it contains shows certain elements are needed to qualify: a well developed storyline synopsis, some character information, a few screenshots of gameplay and in some cases, a demo. Other useful attributes for a project post to have are a linkto any external development website or gameplay video. It is worth noting that only an Admin or Moderator can move a project from the New Projects board to the Substantial Projects board.