• 062.07.04.01: Performance-based navigation (PBN) principles
• 062.07.04.02: On-board performance monitoring and alerting

When flying under an RNP (Required Navigation Performance) PBN specification, the navigation system is required to continuously calculate/estimate the Actual Navigation Performance (ANP), which is technically called the Total System Error (TSE), and is the geometric sum of the:

• Path Definition Error (PDE) - How closely the path in the FMS resembles the intended path.
• Flight Technical Error (FTE) - How closely the aircraft follows the path in the FMS.
• Navigation System Error (NSE) - The accuracy of the navigation system (difference between estimated position and true position).

The TSE is estimated at all times during flight, and shown to the pilots as the ANP (or EPE, TSE, etc. depending on system). If the ANP is less accurate than the RNP for more than 5% of the time, the pilots will be alerted, and the navigation performance is not good enough to use that navigation specification.

In terms of the wording of this question, the navigation "accuracy" is defined as the TSE, which is the difference between the intended (required) position and the true position. You can see the make-up of the errors involved in the annex above.