The AWS FSx service allows you to create an external file system that can be attached to an EC2 instance for supplementary storage.

If no subnet is selected, AWS will pick the default subnet within the selected VPC. Once your subnet is identified, the next step will be identifying an Availability Zone.

There are any number of tasks within AWS that require the use of subnets. For example, you can't create an EC2 virtual machine (VM) without first defining a subnet within which the VM will operate.