What is get_measured_joint_forces?

I am using get_measured_joint_forces and trying to calculate the stress on the joint,
however I need clarification :

  1. if the function measuring force from link on joint?
  2. Am I understand correctly that get_measured_joint_forces read eternal force applied onto joint, while the get_applied_joint_efforts reads force applied by joint drive?
  3. what is the direction convent of the force? is it respect to world frame or local frame?
    Thank you