Splines can be used for

Splines can be defined and edited as well as saved and restored from disk. Further, the splines have such as visibility, line width, and color in order to more easily select and manipulate them.

The splines are defined as piece-wise cubic. The spline is therefore always guaranteed to pass through the control points which you define. The definition of the control points can come from a part (such as a particle trace perhaps), be picked in the graphics window, be input via x/y/z coordinates, or from the position of the cursor tool.