This device is designed to monitor your heart rate during pool swimming sessions. It is compatible with several Garmin models. More details here:
https://buy.garmin.com/en-US/US/p/512311

It can be used for
Pool swims
Open water swims
Triathlon competition (pool swim)
Runs or bike rides as part of a multi-sport activity that includes a pool swim.

Bought a watch bundle but do not need this sensor. Item is new and unwrapped.