The "break line" is a generic term, used to indicate a sharp discontinuity in the slope of the TIN surfaces. The triangles in the TIN are generated in such a way that no triangle crosses a breakline.
A "feature line" is a Civil-3D design element. It is like a lightweight alignment+profile. The differences between a Feature Line and an Alignment/Profile are:
- Alignments/Profiles have stationing that the user can control. Featurelines always have station 0+00 at one end.
- Alignments/Profiles are created and edited using different tools than Feature Lines, making it easier to use one for certain things, and easier to use the other for other things.
- Alignments/Profiles can have vertical curves, superelevations, design speeds, etc., while Feature Lines cannot.
- Alignments/Profiles can be used as Corridor baselines, while Feature Lines can be used as the basis for Grading objects.
- Alignments/Profiles have different labeling features than Feature Lines.
- A single Alignment can have multiple Profiles, while a Feature Line is more like a cross between a standard polyline and a 3D-Polyline. A Feature Line is basically a 3D-Polyline that can have curves in its horizontal layout.
A Feature Line can be used as a breakline in a surface, but there are other things that can also be used as breaklines.
I'm sure others will chime in if I missed anything...
