This is a map to practice defending against MMM attacks. You can create any Zerg army and specify the makeup of the attack.
The map is a bit like the challenge maps but you have much more options. You start with the southern main/natural/highground bases on metalopolis, and you can quickly create your army and specify how much marines/marauders/medivacs you will encounter with what upgrades. When you press ready, the terran army will spawn at the high yeald and attack your base.
To create your army you can instantly get any upgrade from the corresponding building or spawn units instantly from a giant overseer called "the spawner" (which dissapears when battle starts).