Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Material Type
GV_MATERIAL_TYPEParameter: Exclude
GV_MATERIAL_EXCLUDE_IDParameter: First Material
GV_MATERIAL_INPUT_STARTParameter: Maximum Materials
GV_MATERIAL_INPUT_COUNTParameter: On
GV_MATERIAL_OPERATOR_ONParameter: Count
GV_MATERIAL_OUTPUT_COUNT