There are currently 6 different types of asset on the Substance 3D Assets platform:
Surfaces: the outward surface appearance of an object, for example the brick wall of a building, or the skin of a character.
Decals: essentially a type of ’sticker’ that you can apply to an object. Examples include stains, marks, holes, and so on.
Atlases: a special material with additional small detail elements – for instance, a forest floor material that includes stones, fallen leaves, and sticks.
Static Models: a 3D shape – for example, a chair, a car, or a bed.
Environment Lights (aka HDRIs): a 360-degree background image which also acts as a light source. If you want your 3D scene to be illuminated by hard sunlight, for instance, you can use an HDRI which creates this effect.
Additional types of assets are available on the Substance 3D Community Assets platform.