Texturing is the addition of a material to a segment face. Although some textures may appear 3-Dimensional, they in fact are not and will not collide with either players or auto-ops. Certain textures (typically rocks) can be made to look more realistic by assigning a 'Deformation Height', triggering an algorithm in the Overload Level Editor that creates random 3D noise on a surface. Aligning textures can be a challenging endeavor, but the Overload Level Editor's UV Editor can assist in lining up edges of these assets with one another across segment faces.
Currently, OLE does not support custom textures. Arne DeBruijn has created a tool to import any non-stock .png files in the editor install's 'LevelTextures' directory into an Overload level. These will appear in the game, but will only be very flat surfaces, as they have no associated depth or shading information in the Unity engine itself. That tool can be found here.