db
A DB subnet group to associate with the DB instance. If you update this value, the new subnet group must be a subnet group in a new VPC. If there's no DB subnet group, then the DB instance isn't a VPC DB instance. For more information about using Amazon RDS in a VPC, see Amazon VPC and Amazon RDS in the Amazon RDS User Guide. This setting doesn't apply to Amazon Aurora DB instances. The DB subnet group is managed by the DB cluster. If specified, the setting must match the DB cluster setting.