The Project tool allows you to project a spline onto a surface or other geometry. See the video below for two possible uses
Here, Jamie shows how simple it would be to create a pipeline on the surface of some terrain by using the PRoject Spline Tool in Cinema 4D and provides some tips for getting some good results. Jamie also provides some tips for using Project Spline in a challenging situation, and how to get results by using Current State to Object. Check out the tutorial for Using the Project Spline Tool in Cinema 4D here.