Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Strength
MGSPLINEEFFECTOR_STRENGTHParameter: Mode
MGSPLINEEFFECTOR_OFFSETMODEParameter: Relative Offset Axis
MGSPLINEEFFECTOR_RELATIVEAXISParameter: Falloff Modifier
MGSPLINEEFFECTOR_FALLOFFSTRENGTHParameter: Spline
MGSPLINEEFFECTOR_SPLINEParameter: Up Vector
MGSPLINEEFFECTOR_UP_VECTORParameter: Rail
MGSPLINEEFFECTOR_RAIL_SPLINEParameter: Target Rail
MGSPLINEEFFECTOR_TARGET_RAILParameter: Offset
MGSPLINEEFFECTOR_OFFSETParameter: Start
MGSPLINEEFFECTOR_STARTParameter: End
MGSPLINEEFFECTOR_ENDParameter: Clamp
MGSPLINEEFFECTOR_CLAMPParameter: Segment Mode
MGSPLINEEFFECTOR_SEGMENTMODEParameter: Segment
MGSPLINEEFFECTOR_SEGMENTINDEXParameter: Seed
MGSPLINEEFFECTOR_SEGMENTSEEDParameter: Transform Mode
MGSPLINEEFFECTOR_TRANSFORMMODEParameter: Position
MGSPLINEEFFECTOR_POSITION_ACTIVEParameter: Rotation
MGSPLINEEFFECTOR_ROTATION_ACTIVEParameter: R
MGSPLINEEFFECTOR_ROTATION