Thank you for the additional information.
If I understand correctly, you were hoping to use the contour at u=0 to create a zone which was a subset of your original zone. (Such as all the cells where u <=0).
Tecplot thinks of a contour as the line separating contour color bands. This is what it will show you in the contour legend. So when you extract a contour (Data/Extract menu) you get zones that look like lines, which is why the elements in such a zone are FE LineSeg. LineSeg elements are just lines - they have no area to fill with color, so there is no way to make a contour plot from them. So another approach is needed here.
Instead of extracting a contour line, we suggest using value blanking to blank out the portion of your original zone you do not want to see. Then you can use the CRSFEZ addon (it is under the Tools menu but it needs to be activated before you can use it) to create a finite element zone which is the remainder of your original zone (after blanking).
Then you could extract the points you wanted where v=0 from your new subzone.
To activate the CRSFEZ addon, close out of Tecplot, and go to the folder where Tecplot was installed. Locate the file called tecplot.add and open it in a text editor. Then find the line that says this:
# $!LoadAddon "crsfez" # Extract finite-element sub-zone
and delete the '#' at the beginning of the line. Then save and close the file and restart Tecplot, so it can reread the tecplot.add file. Then the CRSFEZ addon will appear in Tecplot's Tools menu. It is easy to use. You need only to adjust the scale of the X and y axes, or just zoom in/out, so the portion of the plot you want to extract is the only thing you see on the screen. Then jst click the addon and it will extract that portion of your zone for you.
Tech Support Lead