Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: MATERIAL_PAGE_PROPERTIES
MATERIAL_PAGE_PROPERTIESParameter: Background Reflection
SHADOWCATCHER_REFLECTION_COLORParameter: Catch Shadows
SHADOWCATCHER_SHADOWSParameter: Shadow Strength
SHADOWCATCHER_SHADOW_STRENGTHParameter: Shadow Color
SHADOWCATCHER_SHADOW_COLORParameter: Include Lighting
SHADOWCATCHER_SHADOW_LIGHTINGParameter: Catch Reflections
SHADOWCATCHER_REFLECTIONSParameter: Reflection Strength
SHADOWCATCHER_REFLECTIONS_STRENGTHParameter: Reflection Mask
SHADOWCATCHER_REFLECTIONS_MASK