Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Display Position
PLANESURFACE_DISPLAY_POS
Parameter: Display Size
PLANESURFACE_DISPLAY_SIZE
Parameter: Plane Type
PLANESURFACE_TYPE
Parameter: Coefficient A
PLANESURFACE_A
Parameter: Coefficient B
PLANESURFACE_B
Parameter: Coefficient C
PLANESURFACE_C
Parameter: Coefficient D
PLANESURFACE_D
Parameter: Infinity Limit
PLANESURFACE_LIMIT