Why do i have a rope tired to the tail with a red triangle of something ? :-)737-800WL /Michaelit has something to do with the flight test package/Michael

That's the flight test option. You can turn it off in the CDU.

how do you use it?/Michael

I think it's purely aesthetic

In reality, it records the static pressure off airplane fuselage effects and, as I believe, is then used to calibrate the static ports. Does not show up in the cocpit though.