A replacement rubber booth to seal the Surge headphone jack & Amphibx armband from any water.
The boot is removable and can be replaced. Since 2015 all the new Surge headphones come with a slim-fit headphone jack. This rubber boot creates a water tight seal that keep the headphone jack (male) and the armband audio jack (female) dry and clean.
It is always recommended to perform a leak-test when you replace the boot to check that the armband is still fully water tight.