The Summary/Legend panel is located at the bottom left of the Merge window. The panel helps to understand the marking and highlighting of elements in both the Merged Result tree/list and the Specification panel.

On the Summary/Legend panel, for 3-way merge, you can see how many differences have been detected between the ancestor and the source, as well as between the ancestor and the target. 

For 2-way merge, you can see how many differences have been detected between two versions, and what kind of changes were detected.