Material ID Overview
The Material ID is assigned to VRay materials and can be used with "Material ID" and "Multimatte" render elements. Its settings can be found on the Options tab of VRay materials.
Use Material ID - specifies whether a material id will be used.
Material ID - this is the color that will be used by the "Material ID" render element.
Multimatte ID - this is the integer ID of the material to be used by the "Multimatte" render element.