The checkbox field in both current OSD and future ROSA cluster provisioning UIs, should change the current field for "Customer Managed Keys" to what it more accurately is about:

"AWS KMS Keys for AWS EBS volume Encryption"

The description / microcopy below it should explain that this enables the use of customer keys in AWS KMS as the encryption keys for their cluster's AWS EBS volumes, used for kubernetes workload (pod) persistent volumes.