When launching an instance with EC2, AWS recommends not to select the availability zone (AZ). AWSspecifies that the default Availability Zone should be accepted. This is because it enables AWS toselect the best Availability Zone based on the system health and available capacity. If the userlaunches additional instances, only then an Availability Zone should be specified. This is to specify thesame or different AZ from the running instances. Reference: http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-regions-availabilityzones.html