The VRayMultiSubTex texture allows the user to control any parameter that can be mapped based on the Object ID or Face ID on which the final material is applied.
Get ID from - specifies the meaning of the ID parameters.
Face material ID - when this mode is selected the VRayMultiSubTex will consider the face IDs of the object when feeding the data (color or texture) to the material.
Object ID - when this mode is selected VRayMultiSubTex will consider the Object IDs of the object when feeding the data (color or texture) to the material.
Default Color - a default color or texture for objects without Object IDs or Face IDs
Num IDs - can be used to set the number of sub-textures.
Delete unused - this button deletes disabled slots from the list.
ID (1-20) - Specifies a color or a texture to be assigned to the material when it is assigned on a object with the current ID.