Designed to snap onto the "D" ring at the top end of your case and will allow you to attach another case (computer, briefcase, etc.) to the outside of your Porter Case or PCX case when it is in the Tote position. Adjust the Exterior Hook to allow your other case to just touch the ground in the upright position. There is a snap buckel mechanism that will grasp your other case with a secure grip. When you rotate your case to roll, the case hooked to the Exterior Stap lifts and goes too. An inexpensive easy way to make your travel more convenient.