The record files this fish as “Westslope cutthroat trout (C&R / slash restrictions)”. That condition is the agency’s and it outranks anything on this drawing.
What the middle band is. growth optimum and the temperature band the authors recommend for thermal protection. A growth optimum is measured on juveniles fed all they can eat and a survey mode is where a net found the most of them; neither is the fish choosing, and both are worth less weight than a preferendum for deciding where to stand today.
What the edges are. No cold edge published. Warm edge: 60-day ultimate upper incipient lethal (19.6 °C); the conventional 7-day figure is 24.1 °C. These are not one consistent quantity across species. A measured avoidance temperature is where fish were seen leaving; a water-quality maximum is a regulatory ceiling written to prevent a kill, and it sits well above the point a day gets difficult.
Source. Bear, McMahon and Zale 2007, Comparative Thermal Requirements of Westslope Cutthroat Trout and Rainbow Trout, Transactions of the American Fisheries Society 136. Read it.
Warmwater species swing their preference ten degrees or more between winter and summer in the same laboratory. One band per species is a summer simplification, and it is why the drawing shows a band rather than a line.