What does getRobotFootprint() refer?

edit

There is a function footprintCost() in carrot_planner.cpp

  double CarrotPlanner::footprintCost(double x_i, double y_i, double theta_i){
    if(!initialized_){
      ROS_ERROR("The planner has not been initialized, please call initialize() to use the planner");
      return -1.0;
    }
    std::vector<geometry_msgs::point> footprint = costmap_ros_->getRobotFootprint();
    //if we have no footprint... do nothing
    if(footprint.size() < 3)
      return -1.0;

    //check if the footprint is legal
    double footprint_cost = world_model_->footprintCost(x_i, y_i, theta_i, footprint);
    return footprint_cost;
  }

The thing I want to know is a part of footprint.size(). I think that costmap_common_params.yaml has footprint like this vector.

footprint: [[0.18, 0.18],[0.18, -0.18],[-0.18,-0.18],[-0.18,0.18]]

So, I have to set above footprint instead of robot_radius. But global_planner doesn't need the footprint.
Am I correct?