Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Spline Object
FIELDLAYER_SPLINE_OBJECTParameter: Spline Shape
FIELDLAYER_SPLINE_SHAPEParameter: Distance Mode
FIELDLAYER_SPLINE_DISTANCEMODEParameter: Radius
FIELDLAYER_SPLINE_DISTANCEParameter: Clip to Shape
FIELDLAYER_SPLINE_CLIPParameter: Radius Falloff
FIELDLAYER_SPLINE_RADIUSCURVEParameter: Segments
FIELDLAYER_SPLINE_ALONGMODEParameter: Range Start
FIELDLAYER_SPLINE_RANGE_STARTParameter: Range End
FIELDLAYER_SPLINE_RANGE_ENDParameter: Offset
FIELDLAYER_SPLINE_OFFSETParameter: Along Curve Falloff
FIELDLAYER_SPLINE_ALONGCURVEParameter: Cycle
FIELDLAYER_SPLINE_ALONGCURVECYCLEParameter: Mask Projection
FIELDLAYER_SPLINE_PROJECTIONParameter: Mask Falloff
FIELDLAYER_SPLINE_MASKFALLOFFParameter: Distance
FIELDLAYER_SPLINE_MASK_DISTANCEParameter: Clip to Shape
FIELDLAYER_SPLINE_MASK_CLIP