Materials can't be assigned to the Voxel World directly. Instead, Surface Types are assigned to individual stamps. A Surface Type is an asset which points to a standard Unreal material asset. It tells Voxel Plugin that material can be used on the terrain.

TIP

When a new Surface Type is assigned for the first time, a pop-up will appear prompting you to add it to the MegaMaterial. Accept the addition. If no MegaMaterial is assigned on the Voxel World, you'll be prompted to create a MegaMaterial. Accept that.

Smart Surface Types (and their parent, Surface Type Graphs) are used to achieve auto-material style behavior, where different Surface Types are automatically applied depending on rules defined in a graph. For example, to apply a cliff material on steep slopes, or a snow material above a certain height.