RPO defines the amount of data an organization can afford to lose when measured against time.

RTO defines the amount of downtime the organization can afford for its services before it becomes operational again.

Both RPO and RTO are defined by time. For example, an organization can have an RPO set to 4 hours and RTO set to 1 hour. This means, it can afford to lose up to 4 hours of data, but it can only afford a service downtime up to 1 hour.

RTO only defines the amount of time a service can remain unavailable but doesn't account for the data loss. This is where RPO pitches in. It defines how much data loss can be afforded.

For example, if you were a company hosting an online document format conversion service, then setting a lower RTO value is very important because the customers will prefer access to the service, rather than to the historical data. The RPO value will determine how much historical data you will have to keep.

Both RPO and RTO help an organization to determine the type of backup and DR solution to meet the business requirements.