What coordinate frame does rviz set the 2D Nav Goal in?

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Hello,

I've got a Turtlebot 2 in ROS Groovy, and I've been learning about navigation. One thing that confuses me is what happens internally in rviz when I press the "2D Nav Goal" button after running roslaunch turtlebot_rviz_launchers view_navigation.launch as done in the tutorial at: http://ros.org/wiki/turtlebot_navigation/Tutorials/Autonomously%20navigate%20in%20a%20known%20map

I know that it sends a goal to the navfn component in the move_base node (I think), but my question is, what reference is it using when I click the screen? IE, if I click on an X,Y point on the screen, what is it passing to move_base? Am I telling the robot to move some distance relative to its base_link? Or am I sending it to a fixed point on its local or global costmap?

Thanks in advance!