Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Global Position
GV_NEARESTPOINTONSPLINE_INPUT_GLOBALPOSITIONParameter: Precision
GV_NEARESTPOINTONSPLINE_INPUT_PRECISIONParameter: Spline Segment
GV_NEARESTPOINTONSPLINE_INPUT_SPLINESEGMENTParameter: All Segments
GV_NEARESTPOINTONSPLINE_INPUT_ALLSEGMENTSParameter: Spline Position
GV_NEARESTPOINTONSPLINE_OUTPUT_SPLINEPOSITIONParameter: Nearest Point
GV_NEARESTPOINTONSPLINE_OUTPUT_NEARESTPOINTParameter: Nearest Segment
GV_NEARESTPOINTONSPLINE_OUTPUT_NEARESTSEGMENTParameter: Tangent
GV_NEARESTPOINTONSPLINE_OUTPUT_TANGENT