"Zone" shows up in at least three unrelated systems while you're shopping for a heat pump. Here's what each one actually governs, so a number on a spec sheet or a calculator doesn't get mistaken for a different one.
The International Energy Conservation Code splits the continental US and Hawaii/Alaska into eight zones, numbered coldest to mildest in the opposite direction most people expect: Zone 1 is the hottest, Zone 8 is subarctic. Each zone also carries a moisture designation (humid, dry, or marine) that affects insulation and moisture-control requirements, but the number alone tells you roughly what dominates your heating and cooling load.
The lower the zone number, the more your equipment choice should weight cooling efficiency (SEER2); the higher the number, the more it should weight heating efficiency (HSPF2) and cold-climate capacity. Zones 6 and above are where a cold-climate-rated model, not just any heat pump, starts to matter, see do heat pumps actually work when it's below freezing.
The Department of Energy and ENERGY STAR both publish IECC climate zone lookups by state and county. Your local building department or a contractor doing a proper load calculation will also know your zone offhand, it's a required input to a real Manual J report, not an optional detail.
The SEER savings calculator on this site covers the Los Angeles and Ventura County area specifically, and its climate-zone presets borrow numbers from California's own Title 24 energy code, which divides the state into 16 zones, a separate, state-specific system from the national IECC zones above. In broad terms, the calculator's lower zone numbers represent milder, coastal-influenced areas with fewer annual cooling hours, and its higher numbers represent inland valley and high-desert areas with substantially more.
California's 16 zones aren't drawn along city or county lines, two neighborhoods a few miles apart can fall in different zones depending on elevation and distance from the coast. If you're outside the LA/Ventura area, or want your exact zone rather than the calculator's regional preset, the California Energy Commission's climate zone map is the authoritative source, use the calculator's presets as a reasonable estimate, not a substitute for it.