If you need to get a tangent to a point/position on a curve that is a Bezier/Nurbs/Spline;
This hip file has an example of how to do that with the primduv function.
Thought I would put it up here for others who have forgotten like me what is meant by 'derivative' in calculus.
Because although I did read the docs description of the function I thought I would have to relearn calculus in order to use it.
After putting in an RFE for a function that would return a tangent they told me this function is the one, so after searching on the net on what derivative means I saw what I had forgot.
So hopefully putting out a post with the functions name it might save time for others who may need it in doing their searches.