A "public" subnet will route traffic to an [InternetGateway]. If you specify a public subnet
this InternetGateway will be created on your behalf and traffic will be routed accordingly.
A "private" subnet is similar to "public" except that the subnet will not have a route to the
[InternetGateway]. Instead, there will be a route entry setup for the NatGateway in that
availability zone to the subnet.
Unlike "public" or "private" subnets, an "isolated" subnet has no routing specified at all.
The type of this subnet.