Cloud SQL zone availability configuration can impact downtime when a zone or instance becomes unavailable.

CLOUD_SQL_ZONE_AVAILABILITY_UNSPECIFIED An unspecified Cloud SQL zone availability.
CLOUD_SQL_ZONE_AVAILABILITY_ZONAL The instance serves data from only one zone. In case of outage, no failover. Not recommended for production.
CLOUD_SQL_ZONE_AVAILABILITY_REGIONAL The instance can serve data from multiple zones in a region. Highly available. Automatic failover to another zone within your selected region. Recommended for production instances. Increases cost.