I'm not sure that SW is being especially secretive about how
they handle splines or curves through points; there may not be a
whole lot to say about it. I suspect that they use splines for the
curves through points. It should be pretty easy to test that out.
Put some xyz coordinates in a spreadsheet and use that to make a
text file and insert a curve through points. Put some points at the
same coordinates and put a spline through the same points. When I
did this for a simple 2D set of points, the spline and curve look
identical.

Splines are nurbs. I suspect that tells the mathematically
competent quite a bit about them.

You are correct, Steiger, in that SW does use splines as
their connecter in Curve Through Points; however SWx does not
disclose how they are mathematically defining their splines.
Splines aren't always defined based on NURBS equations. It really
depends on the accuracy required in your model as to whether or not
you can accept a spline as a substitute for a closed form equation
of a line. I work in optics and using splines does not get us the
accuracy we need. Exact mathematical solutions are required which
unfortunately makes using splines a "no-no" for me.

Splines are nurbs. I suspect that tells the mathematically competent quite a bit about them.