I made a post in meta for adding flags to projects. It includes some links to the various types of projects I’ve been seeing here. I’m not sure what the policy is for some of them, for instance, people selling materials or, less obviously off-topic, people selling their own creations.
My impression is that the Projects | Inventables section is for people to share how they made something, not just what they made; it’s Instructables instead of Pinterest. I’m pretty new around here though, so perhaps my impression is incorrect.