Its supposed to have 4.
Survival: Infinite waves of zombies, see how long you can last.
Infection: When you die, you become a zombie yourself.
Man VS Tank: All players are pitted against a Player Zombie Tank.
Versus: All players are split into two groups, Survivors and Zombies. No bots used.
-snip- Instead of just the constant onslaught, a waves mode.
That's what a Mob is. Use mob spawns to make a mob. Mobs are a set amount of zombies made every set amount of time. So, every say, 30 seconds, 15 zombies instantly spawn instantly. The time interval along with mob size can be edited in the server options...
ZAPT is a perfect mod.