Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Switch To Field
FALLOFF_SWITCHTOFIELDParameter: Field
FIELDSParameter: Shape
FALLOFF_MODEParameter: Invert
FALLOFF_INVERTParameter: Visible
FALLOFF_VISIBLE