First-party material is anything oneself comes up with. (Like first person.)
Second-party material is anything someone else at the gaming table comes up with. (It's first-party to them, of course.)
Third-party material is anything someone not at the table comes up with: random blogs, publicly traded Seattle corporations, an angel gives you golden tablets inscribed with elfgame combat rules, whatever.
This is probably like DVORAK in that it's not worth the switching costs. But it's how I think of it in my head, and certainly the right mindset for DIY elfgames.