Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Display Position
PLANESURFACE_DISPLAY_POSParameter: Display Size
PLANESURFACE_DISPLAY_SIZEParameter: Plane Type
PLANESURFACE_TYPEParameter: Coefficient A
PLANESURFACE_AParameter: Coefficient B
PLANESURFACE_BParameter: Coefficient C
PLANESURFACE_CParameter: Coefficient D
PLANESURFACE_DParameter: Infinity Limit
PLANESURFACE_LIMIT