The RPO (Recovery Point Objective) defines the maximum tolerable data loss, measured in time. An RPO of four hours means data generated in the four hours before a failure may be lost. The RPO directly determines backup frequency. For critical systems with an RPO close to zero you need synchronous replication or continuous data protection. In your ISMS you set the RPO per system in the business impact analysis. Together with the RTO, it forms the foundation of your disaster-recovery planning.