Zone 2
Zone 2 is the second of five commonly used training bands, roughly 60–70% of maximum heart rate — an effort you could hold while talking in full sentences. The zone boundaries themselves are a field convention, and different bodies draw them differently.
Example
For someone with an estimated HRmax of 173, zone 2 falls around 104–121 bpm. Because HRmax itself carries about ±10 bpm of uncertainty, so does the zone.