This is the amount of data required to be maintained or restored in order to restore acceptable functionality:

The concept you're looking for relates to the Recovery Point Objective (RPO). RPO is a critical metric in disaster recovery and business continuity planning. It defines the maximum age of the data that can be lost in the event of an incident. Essentially, it indicates how much data must be recovered to return to a point of acceptable functionality after a disruption.

When a business experiences a data loss or system failure, the RPO helps determine how recent the backup must be to ensure minimal disruption to operations. For instance, if the RPO is set to four hours, it means that backups need to be taken regularly enough that, in the worst-case scenario, no more than four hours of data is lost. This directly informs decision-making around backup frequency and disaster recovery strategies, ensuring that an organization can resume operations with acceptable data integrity and functionality.

Other terms like RTO (Recovery Time Objective), which indicates how quickly you must restore operations, and RSL (Recovery Service Level) or RTL (Recovery Time Length), which are less commonly used, do not define the amount of data required for restoration in the same way RPO does. Thus, focusing on RPO provides a clear understanding of data recovery needs in the event of system failure.