The Unused Cables listbox in the Assign Cable dialog contains both unused cables and cables in inventory. The unused cables are listed first. To reuse an existing cable for a connection, select it by clicking on it in the Unused Cables list box and then press OK. (Alternatively, type its cable ID in the appropriate text box, and press OK).
For verification of unique cable IDs, press the Cable ID's... button to display a list of all cable IDs already in use by connections.
When you change the cable assignment for a connection, the old cable is placed in the unused cables pool.