Hosting a co-op level from the Map screen (CO-OP CAMPAIGN, a level and its
difficulty, then Server Setup) makes a network game on the campaign map
with no game engine, which is what co-op is. Only the host runs the
scripts and the AI; port/linux/game/network_coop.c sends clients what the
scripts do (cutscenes, camera, fades, HUD, sounds, devices, named objects,
animations), and network_actors.c drives the host's AI units on every
machine. Players spawn beside each other, respawn beside a safe teammate,
and spectate while dead or waiting. Skipping a cutscene is a vote.
Players can join a game in progress at any time.