Not sure about the part that you circled, but I popped up the red cover on the terminal, removed the nut, installed my own cable with a ring connector on the post and screwed on the nut again. Don't think I ever had to remove the cable that is on the post. Lastly, put the red cover back on.
For what its worth, I pulled the sub that I installed out after a missing software package was installed that addressed the lacking bass in a SSP equipped vehicle. I may reinstall it at a later date, but need to figure out how to mount this under the seat powered sub to the back of the rear seat in the trunk. I tried 3M double sided tape made for automotive and velcro. The heat in the trunk would soften the adhesive and the sub would fall off the velcro.