The 3dsmax UI only allows you to set material effects ID up to 16 channels. The VRayMtl allows you to go beyond this using the effects ID override but other types of materials don't have any UI to adjust this. This tool using a backdoor into 3dsmax to set the effects ID beyond the 16 limit imposed by the UI.
This means you can create masks for every material in your scene. Using this tool you can either assign effects IDs by hand or just press to automatically create them and their mask render elements.
You can also make a Vray Multi-matte mask for each layer in the scene too, this uses the include/exclude lists rather than the GBuffferIds, so that you can therefore use the GBuffer IDs for doing variation with texture maps or mask objects individually if you want to.
Bug found with VRay that made some of the mask render white. Put work-around in the script, don't worry that the layer masks say their ID is 10000.