What is the black strap with red plastic piece off the back/bottom for?

Hi Joanne, thank you for your enquiry, the black strap with the red plastic is the seat belt guide for when the seat is turned into a half booster. We hope this helps.  if you follow the link https://www.babyonthemove.co.nz/store-locations/ this will take you to our online store finder. Here you can find contact details for your nearest Baby On The Move Store where one of our team experts will be happy to assist you in your product search and help answer any further questions you may have. We hope this has helped with your query.