The system graph is an abstract map of the system, laying out where each rail and station is located.
In installations with blocking buffer calls, the graph needs to be connected for all lines that will be used for blocking calls. If this is not done, the blocking calls will still work, but they will be very slow, since the system must wait for each carrier to arrive at the destination before sending out the next carrier.
If blocking calls are not going to be used, setting up this part is optional.