c4d_base/description/zmultipass.hParameters are accessed with cinema::C4DAtom::GetParameter() and cinema::C4DAtom::SetParameter(). See Parameters.
Parameter: Group ID
MULTIPASSOBJECT_OBJECTBUFFERParameter: AOV ID
MULTIPASSOBJECT_AOVBUFFERParameter: Ambient
MULTIPASSOBJECT_AMBIENTParameter: Diffuse
MULTIPASSOBJECT_DIFFUSEParameter: Specular
MULTIPASSOBJECT_SPECULARParameter: Shadow
MULTIPASSOBJECT_SHADOWParameter: Reflection
MULTIPASSOBJECT_REFLECTIONParameter: Refraction
MULTIPASSOBJECT_TRANSPARENCYParameter: Global Illumination
MULTIPASSOBJECT_RADIOSITYParameter: Caustics
MULTIPASSOBJECT_CAUSTICSParameter: Atmosphere
MULTIPASSOBJECT_ATMOSPHEREParameter: Atmosphere (Multiply)
MULTIPASSOBJECT_ATMOSPHERE_MULParameter: Post Effects
MULTIPASSOBJECT_ALLPOSTEFFECTSParameter: Separate Materials
MULTIPASSOBJECT_REFLECTION_MATERIALSMULTIPASSOBJECT_REFLECTION_SEPARATE_NONEMULTIPASSOBJECT_REFLECTION_SEPARATE_ALLMULTIPASSOBJECT_REFLECTION_SEPARATE_SELECTEDParameter: Separate Layers
MULTIPASSOBJECT_REFLECTION_LAYERSMULTIPASSOBJECT_REFLECTION_LAYERS_NONEMULTIPASSOBJECT_REFLECTION_LAYERS_ALLMULTIPASSOBJECT_REFLECTION_LAYERS_SELECTEDParameter: Mode
MULTIPASSOBJECT_REFLECTION_MODEMULTIPASSOBJECT_REFLECTION_MODE_NORMALMULTIPASSOBJECT_REFLECTION_MODE_RAWMULTIPASSOBJECT_REFLECTION_MODE_COMBINEDParameter: Separate Materials
MULTIPASSOBJECT_SPECULAR_MATERIALSMULTIPASSOBJECT_SPECULAR_SEPARATE_NONEMULTIPASSOBJECT_SPECULAR_SEPARATE_ALLMULTIPASSOBJECT_SPECULAR_SEPARATE_SELECTEDParameter: Separate Layers
MULTIPASSOBJECT_SPECULAR_LAYERSMULTIPASSOBJECT_SPECULAR_LAYERS_NONEMULTIPASSOBJECT_SPECULAR_LAYERS_ALLMULTIPASSOBJECT_SPECULAR_LAYERS_SELECTEDParameter: Mode
MULTIPASSOBJECT_SPECULAR_MODEMULTIPASSOBJECT_SPECULAR_MODE_NORMALMULTIPASSOBJECT_SPECULAR_MODE_RAWMULTIPASSOBJECT_SPECULAR_MODE_COMBINED