There is a difference between power and a Heartrate. Your power is for example the watts you produce when your are cycling.
You can also set a zone for a power zone:
let powerMinValue = HKQuantity(unit .watt(), doubleValue: 250)
let powerMaxValue = HKQuantity(unit .watt(), doubleValue: 270)
let powerRange: WorkoutTargetType = .range(min: powerMinValue, max: powerMaxValue)
let powerAlert = WorkoutAlert(type: .currentPower, target: powerRange)
This can be seen on on this WWDC video