How Relative Feature Lines Work
Relative Feature Lines in Site Grading
Jeff Bartels of Autodesk explains the basic mechanics of Surface Relative Feature Lines in Civil 3D 2018 in one video
and delivers a demo of Feature Line usage in a parking lot grading senario in a second.
From the Jeff Bartels YouTube Channel.
and the Civil Imersion blog.
Note the workflow that includes an intermediate temporary surface, Feature Lines, and Corridors based on Feature Line Baselines.
If you are going to do this in a real project, you probably want to employ a structure of DREF Features to better manage the end results.