WHOOP defines Zone 2 as 60-70% of your realized maximum heart rate, or the maximum heart rate that you have logged using WHOOP. This range ensures that you remain at the correct metabolic and effort level to gain near-maximum adaptation while mitigating fatigue that could hurt future performance.
The definition of Zone 2 is a hazardous area classified as an atmosphere where a mixture of air and flammable substances in the form of gas, vapour or mist is not likely to occur in normal operation, but if it does occur, will persist for a short period only.