Zones are separated into subzones by minimum average temperature variants of 5 degrees. So while Zone 10 has a minimum annual temperature of 30-40 degrees, Zone 10a ranges from 30-35 degrees and Zone 10b ranges from 35-40 degrees. It provides a bit more specificity to your growing zone and is meant to be a guideline for planting. All zones are separated into two subzones and broken down further by minimum annual temperature.