You’ll use the ASCII 3D to Feature Class tool to import breaklines. Breaklines are linear features, such as roads and water body shorelines, that need to be represented on the surface. These are provided in the 3-D GENERATE format; an ASCII format similar to XYZ but more appropriate for line and polygon data. You can find a detailed description of the format in the help. To find it, click the Learn more about link located at the top of the help page for the ASCII 3D to Feature Class tool.