In the API command ReadNextSegment, is pathLength = optical path length or physical path length?

ReadNextSegment (out int segmentLevel, out int segmentParent, out int hitObj,
out int InsideOf, out double X, out double Y, out double Z, out double L, out
double M, out double N, out double exr, out double exi, out double eyr, out
double eyi, out double ezr, out double ezi, out double intensity, out double
pathLength)
 

Where is the documentation on this?

Is there a way to select optical or physical with this command?