Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Spline Object
FIELDLAYER_SPLINE_OBJECT
Parameter: Spline Shape
FIELDLAYER_SPLINE_SHAPE
Parameter: Distance Mode
FIELDLAYER_SPLINE_DISTANCEMODE
Parameter: Radius
FIELDLAYER_SPLINE_DISTANCE
Parameter: Clip to Shape
FIELDLAYER_SPLINE_CLIP
Parameter: Radius Falloff
FIELDLAYER_SPLINE_RADIUSCURVE
Parameter: Segments
FIELDLAYER_SPLINE_ALONGMODE
Parameter: Range Start
FIELDLAYER_SPLINE_RANGE_START
Parameter: Range End
FIELDLAYER_SPLINE_RANGE_END
Parameter: Offset
FIELDLAYER_SPLINE_OFFSET
Parameter: Along Curve Falloff
FIELDLAYER_SPLINE_ALONGCURVE
Parameter: Cycle
FIELDLAYER_SPLINE_ALONGCURVECYCLE
Parameter: Mask Projection
FIELDLAYER_SPLINE_PROJECTION
Parameter: Mask Falloff
FIELDLAYER_SPLINE_MASKFALLOFF
Parameter: Distance
FIELDLAYER_SPLINE_MASK_DISTANCE
Parameter: Clip to Shape
FIELDLAYER_SPLINE_MASK_CLIP