Lines appear other than centerline

Operating system:
Slicer version:4.11
Expected behavior: Only one line as centerline
Actual behavior:getting another colorful network such as a graph

I am using the tutorial to get the center line of the vessels attached to a bigger organ with the following settings :

In the output I get one green line which is the center line but I also get another colorful network such as a graph and the edges of that graph is sometimes connected to each other from out of the mesh. What are those lines are exactly and why they are colorful?

The Extract centerline module’s documentation should answer most of your questions. If anything is not clear then let us know. To make sure we point you to the right direction, please also describe what your clinical end goal is (how do you plan to use the extracted centerlines).