availabilityZone

For regional NAT gateways only: The Availability Zone where you want to associate an Elastic IP address (EIP). The regional NAT gateway uses a separate EIP in each AZ to handle outbound NAT traffic from that AZ.

A regional NAT gateway is a single NAT Gateway that works across multiple availability zones (AZs) in your VPC, providing redundancy, scalability and availability across all the AZs in a Region.