Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Mirror Plane
SYMMETRYOBJECT_PLANE
Parameter: Weld Points
SYMMETRYOBJECT_WELD
Parameter: Weld Tolerance
SYMMETRYOBJECT_TOLERANCE
Parameter: Symmetrical
SYMMETRYOBJECT_ONPLANE
Parameter: Clamp Points on Axis
SYMMETRYOBJECT_CLAMPPOINTS
Parameter: Delete Polygons on Axis
SYMMETRYOBJECT_DELETEPOLYGONS
Parameter: Automatically Flip
SYMMETRYOBJECT_AUTOFLIP
Parameter: Flip
SYMMETRYOBJECT_FLIP
Parameter: Convert to New Symmetry Generator
SYMMETRYOBJECT_CONVERT_LEGACY