These are the USDA’s Hardiness Zones. According to this map the Salt Lake Valley falls into zones 5/6. When choosing plants to purchase you can use this as a general guideline. However, when looking up a plant in any Sunset books you will need to reference a different map:
Unlike the USDA map which only uses minimum winter temperatures for climate zones, Sunset uses many other factors such as summer highs, elevation, rainfall, humidity, and the growing season. Under Sunset’s assignment, the Salt Lake Valley is Zone 3. THIS IS NOT THE SAME AS A USDA ZONE 3. Sunset uses their own separate numbering system.