Uncheck this if you want the material does not affect raycasting. In other word unchecking this the material become "transparent" to projectiles, raycasters and all raycasting gameMachine actions.
This is a combo box containing all possible material archetypes.
The archetype is used when the material affects raycasting, for example it is hit by a projectile or a raycaster.
In this case projectiles and raycasters check the archetype of the material for producing the right particle system and decal for simulating the impact.
Projectiles and RayCasters have parameters containing tables that make correspondence between the material archetype and the particle system or decal to spawn.
For example if a material has a MAT_METAL archetype and the projectile fired by player's gun has Sparks particle system into the MAT_METAL archetype row of its archetypes table for particle systems, when the material is hit the Sparks particle system is spawned.
See Projectile for more info about raycast collisions.
The following is the complete list of material archetypes:
