For information about valid Iops values, see Amazon RDS Provisioned IOPS Storage to Improve Performance in the Amazon RDS User Guide. Constraints: For MariaDB, MySQL, Oracle, and PostgreSQL DB instances, must be a multiple between .5 and 50 of the storage amount for the DB instance.
https://docs.aws.amazon.com/AmazonRDS/latest/APIReference/API_CreateDBInstance.html