Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Spline
SLA_SPLINE_OBJECTParameter: Background Texture
SLA_SPLINE_BACKGROUND_COLORParameter: Line Texture
SLA_SPLINE_LINE_COLORParameter: Fill Texture
SLA_SPLINE_FILLET_COLORParameter: X Offset
SLA_SPLINE_X_OFFSETParameter: Y Offset
SLA_SPLINE_Y_OFFSETParameter: X Scale
SLA_SPLINE_X_SCALEParameter: Y Scale
SLA_SPLINE_Y_SCALEParameter: Plane
SLA_SPLINE_USE_PLANEParameter: Single Pixel
SLA_SPLINE_SINGLE_PIXEL_WIDTHParameter: Line Width
SLA_SPLINE_LINE_WIDTHParameter: Smooth Width
SLA_SPLINE_SMOOTH_WIDTHParameter: Caps
SLA_SPLINE_HAS_CAPSParameter: Fill
SLA_SPLINE_FILLParameter: Use Fill Color
SLA_SPLINE_FILL_COLORParameter: Bump Width
SLA_SPLINE_BUMP_WIDTHParameter: Text Spline
SLA_SPLINE_TEXTParameter: Text
SLA_SPLINE_TEXT_TEXTParameter: Font
SLA_SPLINE_TEXT_FONTParameter: Align
SLA_SPLINE_TEXT_ALIGNParameter: Height
SLA_SPLINE_TEXT_HEIGHTParameter: H Spacing
SLA_SPLINE_TEXT_HSPACINGParameter: V Spacing
SLA_SPLINE_TEXT_VSPACING