Amazon Virtual Private Cloud (VPC) allows you to create a subnet of Amazon Elastic Compute Cloud (EC2) instances that can act as your own private network in the cloud. You can work with this subnet independently in the cloud or use it with a VPN connection to expand your organization's internal network.
You need to first create the subnet using the AWS Management Console or one of the AWS APIs. When you create an ArcGIS Server site, Cloud Builder detects the VPC subnets associated with your Amazon account and displays them in a drop-down list on the Amazon Web Services panel. You can then select a subnet and launch your site within it.