rrt_exploration : tree starting point

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Hello everyone, i'm currently testing this package (rtt_exploration) in Gazebo, in order to include it in a future project, and I have a question concerning this paragraph :

clicked_point (geometry_msgs/PointStamped Message): The global_rrt_frontier_detector node requires that the region to be explored is defined. This topic is where the node recieves five points that define the region. The first four points are four defining a square region to be explored, and the last point is the tree starting point. After publishing those five points on this topic, the RRT will start detecting frontier points. The five points are intended to be published from Rviz using alt text button.

I didn't understand what the last point's purpose, and how can I place it efficiently ?

thank you for your help !