Ideas > pitching an idea you do intend to work on but want feedback. the second sticky explicitly stating what the forum is for does specifically state that there is supposed to be actual intent to work on it
Ex: I want to make a dungeon for X character. What other projects can I refer to for inspiration? I know what I want for my final boss, but what kind of mob enemies should it have, etc
Submissions > when you already have progress in your project and/or your project is already completed so you're putting it here for documentation
Ex: I've been working on this dungeon and here is the outline and WIPs for it and some of the enemies.
imo in practice Ideas ends up being a bit of a filter. people that go there, pitch their idea, then never keep working on it are therefore not clogging up this forum with dead threads. the only problem being, what happens if you originally post in Ideas, then actually bring a project to fruition, do you bother reposting in events or should you just continue using the Ideas thread to keep all the brainstorming/feedback consolidated?