AWS: What is the difference between Burst Balance and EBS IO Balance metrics?

AWS Docs describe Burst Balance and EEBS IO Balance in the following way:

BurstBalance The percent of General Purpose SSD (gp2) burst-bucket I/O credits available.

EBSIOBalance% The percentage of I/O credits remaining in the burst bucket of your RDS database. This metric is available for basic monitoring only. This metric is different from BurstBalance

However, as far as I know, the docs do not explain how those two metrics are different.

BurstBalance is an EC2 / EBS metric (EBS falls under the EC2 namespace). EBSIOBalance% is an RDS metric. Under RDS you can't see the EBS volume burst balance directly which is why RDS has to expose it.