Missions in Guild Wars are labelled with one of the following four icons, to indicate the type of enemy generation inside the mission.

Kill All - The missions starts with a set number of walkers, and the goal is to kill all of them.

Threat Counter - The mission has a threat counter that counts down to the turn when walkers would spawn. The number of walkers that will spawn can increase if you use ranged weapons, or decrease if you use weapons or traits with Threat Reduction.

Body Piles - The mission contains piles of bodies, from which a set number of walkers come out every turn. This is unaffected by ranged weapons or Threat Reduction.

Human Enemy - The mission is fought against survivors of an opponent player. Walkers spawn via the Threat Counter in these missions.