What is "CreateDBInstance:0014" and "CreateDBInstance" API operations for "RDS:ChargedBackupUsage (GB-Month)" usage type?

The subject and body appear to be asking 2 different questions. I assume you're interested in knowing why RDS:ChargedBackupUsage is untagged. I am aware of 2 possible cases: 1/ orphaned snapshots have no DB instance to refer to, hence no reference to instance tags either. In Cost Explorer and Cost and Usage Report, costs for RDS orphaned snapshots aggregated in a single, untagged item. 2/ Short-lived instances (less than one hour) might not get time to export tags. As a result, tags might not get applied to snapshots of short-lived instances.

If you have an AWS support option, I recommend opening a support case and support engineer can look into your AWS account and help trace those untagged charges back to specific snapshots / instances.