A Ferguson Zone is a classification (1 to 4) of how much UVB a reptile species needs, based on how much sun it basks in the wild — measured as a target UV Index (UVI) at the basking spot.
Zone 1 is shade-dwellers (UVI ~0–0.7), Zone 2 partial-sun (~0.7–1.0), Zone 3 open-basking (~1.0–2.6) and Zone 4 mid-day sun-lovers (~2.6–7.4+). Matching your bulb and its distance to the species' zone is how you give the right UVB without over- or under-dosing.
The system comes from Baines et al. (2016) and is the modern standard for reptile lighting.