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lookupTransform() is a lower level method which returns the transform between two coordinate frames.

In below code nothing is calculated, just reading the transform between frames "base_footprint", "odom_combined"

As per your above description, odom_combined fame similar to world frame and base_footprint is similar to odom .

  //record the starting transform from the odometry to the base frame
  listener_.lookupTransform("base_footprint", "odom_combined", 
                            ros::Time(0), start_transform);

lookupTransform() is a lower level method which returns the transform between two coordinate frames.

In below code nothing is calculated, just reading the transform between frames "base_footprint", "odom_combined"

As per your above description, odom_combined fame similar to world frame and base_footprint is similar to odom .

  //record the starting transform from the odometry to the base frame
  listener_.lookupTransform("base_footprint", "odom_combined", 
                            ros::Time(0), start_transform);