This is from SR2 manual: Your aerobic fitness level is defined as VO₂max (maximal oxygen consumption), a widely recognized measure of aerobic endurance capacity. In other words, VO₂max shows how well your body can use oxygen. The higher your VO₂max, the better you can use oxygen. The estimation of your fitness level is based on detecting your heart rate response during each recorded running or walking workout. To get your fitness level estimated, record a run or walk with a duration for at least 15 minutes while wearing your Suunto Race 2.
For a walk under certain conditions SR2 should provide VO₂max estimation. Even if not precise as in lab, for someone at the beginning of fitness journey it could be a good indicator of progress and also a motivation.