Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Spline Path
ALIGNTOSPLINETAG_LINK
Parameter: Rail Path
ALIGNTOSPLINETAG_RAIL
Parameter: Tangential
ALIGNTOSPLINETAG_TANGENTIAL
Parameter: Position
ALIGNTOSPLINETAG_POSITION
Parameter: Segment
ALIGNTOSPLINETAG_SEGMENT
Parameter: Axis
ALIGNTOSPLINETAG_AXIS