Campaign matchmaking. Players now will be able to find friends or link up with strangers and fight cooperatively through the game's campaign mode.
"Totally Unrelated Question 4: Can I haz Campaign Matchmaking?" writes Bungie. "Totally Related Answer: Yes."
No other details.
What this means: Halo 3 supported multiplayer campaign but not matchmaking for it. So you could hop into a game with a friend over Xbox Live but couldn't search for them. Halo: Reach will instead feature its own campaign playlist, allowing you to find your pals (or team up with anyone at random.) This will be especially handy for later levels or harder modes where you need a big friend to help out.