When the character is created one of the required components is the LayerManager. The LayerManager specifies which layers the character should use for the specified categories:

Enemy Layers

Layer Mask that specifies the layer that the enemies use.

Invisible Layers

Layer Mask that specifies any layers that are invisible to the character (such as water or invisible planes placed on top of stairs).

Solid Layers

Layer mask that specifies any layers that represent a solid object (such as the ground or a moving platform).

An example use is for the collision detection by the character controller. The controller will use the Solid Layers mask to determine what is considered the ground to prevent the character from falling through the floor.