When working with spheric coordinates, all distance calculations are performed using great circle distances and the Earth is considered a perfect sphere with radius 6378130 meter.
When working with cartesian coordinates, all distance calculations are performed using straight Pythagoras formula. Several different cartesian units are supported. See property Coord for a full list.
It is worth noting, that RW Net never performs any transformation between coordinate systems. It always works with the native coordinates of the base dataset used when creating the network. It will return strange results, if you set the coordinates as spheric, while they are really meters or vice-versa. It is YOUR responsibility to make sure this is correct.