The index of the sub-mesh to combine with the other CombineInstance meshes from the CombineMeshes list.
The Mesh.CombineMeshes API uses this property and CombineInstance.mesh to decide what to combine from this CombineInstance. To ensure all sub-meshes of a specific mesh are included in the combined mesh, assign a separate CombineInstance for each sub-mesh.
Accepted values range from greater than or equal to 0 and less than the sub-mesh count for the corresponding CombineInstance.mesh. The default value is 0.
Additional resources: Mesh.CombineMeshes, mesh.