When you connect two components with an edge, it is red and dashed.
After assigning metadata to the edge, it becomes solid and gray.
Edges with propagated metadata are grey and dashed.
When you click any metadata item in the Outline pane, all edges with the selected metadata become blue.
If you click an edge in the Graph Editor, the selected edge becomes black and all of the other edges with the same metadata become blue. (In this case, metadata are shown in the edge tooltip as well.)
Figure 32.4. Metadata in the Tooltip