How TIN Line (3D Analyst) works
TIN Line writes the hard and soft breaklines from a triangulated irregular network (TIN) into a new 3D polyline feature class. The output lines are assigned codes as a form of attribution to indicate their type: 1 for soft and 2 for hard. The codes are written to the Code Field parameter, which is added to the feature class. The default name for Code Field is Code.
Breaklines in a TIN are defined by triangle edges that are enforced in the triangulation. Consecutively ordered enforced edges of the same surface feature type (SFType) that meet at common nodes, are line segments that make up polylines. The TIN Line tool strings together these connected segments into polylines. Junctures are defined where more than two enforced edges meet at the same node. Junctures are treated as breakline endpoints.