“To identify the location of your resources relative to your accounts, you must use the AZ ID, which is a unique and consistent identifier for an Availability Zone. For example, use1-az1 is an AZ ID for the us-east-1 Region and it is the same location in every AWS account”
So each separate AWS account will have a different AZ name that maps to use1-az1, but use1-az1 is a region-wide constant.