A complete replacement kit in case of damage to the chest strap.
The most often damage occurring within the chest strap are cracks affecting plastic parts (buckles). In order to place the chest strap you must first pry open plastic parts fixing the chest strap to shoulder straps. Then you can replace the strap with a new one, appropriately weaving its ends through the buckles.
The red signalling whistle belong to the kit as its important part.