Gets or sets the fourth corner index of the mesh face. If D equals C, the mesh face is considered to be a triangle rather than a quad.
The structure has 4 int fields, and ints are 0 by default. … but 0 is a valid value for vertex-index.
… so what to do for triangles ? I assume it is wise a design decision from the developer, to repeat the value - this will avoid to handle separate cases for triangles and quads in many situations… as a very simple example if you want to calculate the circumference of the face - you will just get a forth edge with length 0…
There is MeshFace.IsQuad() and MeshFace.IsTriangle() to get a more readable differentiation.
for more general stuff - to store the information “index not found” / “index does not exist” -1 is a nice value to use.