From Rigs of Rods Wiki
A sub-mesh is a list of polygons as defined by three nodes. Due to the nature of a triangle, the three nodes are always coplanar and thus, using a sub-mesh it is impossible to create curves. However, depending on the resolution of the beam and node model of the truck it is possible to make smooth contours in the body of the truck/load/boat, etc. It is possible if using a material file to blend surface textures into one another, but it will still be a solid edge. For even smoother corners a mesh is more effective.
Sub-meshes can be used to help with concepts such as the sliding rail technique, and also roll-on surfaces of certain trucks and other vehicles.
This tutorial aims to provide a fool-proof guide to sub-meshing. It will be in an easy step by step format and will only use notepad and the editorizer.
To start with, we'll break down the sub-mesh section and describe what each value affects and how it will affect the final visual model in-game.
WORK IN PROGRESS LONDEX