It is to clip onto a normal or Fieldsafe headcollar to hold the front leather strap on the muzzle. Don't think you need to use it if you have got the Greenguard Headcollar.
Oh.... thanks. That's good because the front strap was slipping around the noseband today and the muzzle wasn't staying in place. So now, could you explain how you clip it on the noseband please?