The Garmin PRO 550 Plus Handheld can be used as a replacement handheld for a PRO 550 Plus system or can be used with a TT15 or TT15 mini collar that was purchased previously. The Garmin PRO 550 Plus handheld can be used as a great add-on to an Alpha system for those that only run 1-3 dogs at distances under 2 miles.