[h=2]Difference between “hard edge” and “soft edge” in Arc's Add TIN Line[/h]
Hard and soft qualifiers for line and polygon feature types are used to indicate whether a distinct break in slope occurs on the surface at their location. A hard line is a distinct break in slope, while a soft line will be represented on the surface as a more gradual change in slope.
See also this KB article, which provides more explanation and graphic examples. You won't really see a visual difference in the TIN itself - it doesn't become apparent "until a quintic interpolator is used to analyze the TIN surface" (basically interpolated to either contours or a surface, or pulling values off connecting lines). One of the example images from the KB article demonstrates this:
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