I'm looking at the Coders Desk Reference for Procedure codes and I'm reading the layman description for CPT® codes 36215-36218 and I'm not understanding what they mean when they refer to the word "order". For instance "In 36215, the catheter travels in a first order artery. In 36216, the catheter travels further to a second order artery." Does the word "order" mean the branches of arteries. For instance the common carotid arteries branches into the internal and external carotid arteries? So would the external and the internal carotid arteries be the second order and the common carotid be the first order?