ROS Navigation Stack, Global Planner, Waypoints

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The tutorial on Global Planner says:

"The global planner is responsible for generating a high level plan for the navigation stack to follow. Given a goal that is arbitrarily far away from the robot, the global planner will create a series of waypoints for the local planner to achieve. "

How are these waypoints represented? Are they (x,y) coordinates? and How can I retrieve the waypoints when Global Planner is planning a path from the start to the goal point? Are they published on a topic?

Thanks