The index of the sub-mesh to be combined with the other meshes.
The Mesh.CombineMeshes method uses this property along with the mesh property to identify the mesh data copy into the combined mesh. If a mesh contains multiple sub-meshes, you must create a separate CombineInstance for each sub-mesh.
The default value of this property is 0.
Accepted values range from greater than or equal to 0 and less than the sub-mesh count for the corresponding mesh.