An Amazon Load Balancer unites multiple Elastic Compute Cloud (EC2) instances under a common web address. The load balancer plays the role of the web gateway in the ArcGIS Server architecture and prevents any one of the GIS servers from being overwhelmed with incoming requests. You can utilize all the GIS server instances under the load balancer to respond to requests.
ArcGIS Server Cloud Builder on Amazon Web Services creates a load balancer when you build your site, and all the instances in your site are placed under it. Also, any instances created by your auto scaling rules are placed under the load balancer. You see the load balancer address when you examine the URL to your site.
If you build your site using the AWS Management Console, you need to create the load balancer yourself and add your instances to it. The suggested workflow is as follows: