Refer to figures.
Performance-based Navigation (PBN)

The inability to achieve the required lateral navigation accuracy may be due to navigation errors related to aircraft tracking and positioning. The three main errors in the context of on-board performance monitoring and alerting are path definition error (PDE), flight technical error (FTE), and navigation system error (NSE).

Path Definition Error (PDE) occurs when the path defined in the RNAV system does not correspond to the desired path, i.e. the path expected to be flown over the ground. RNAV system is unable to accurately define the desired path. Path definition error (PDE) is constrained through database integrity and functional requirements on the defined path, and is considered negligible.

Flight Technical Error (FTE) relates to the air crew or autopilot’s ability to follow the defined path or track, including any display error (e.g. course deviation indicator (CDI) centring error). On-board performance monitoring and alerting of Flight Technical Error (FTE) is managed by on-board systems (i.e. autopilot) or crew procedures (i.e. SOPs).

Navigation System Error (NSE) refers to the difference between the aircraft’s estimated position and actual position. It is the error in the accuracy in determining actual aircraft coordinates.

The figures show two FMS pages - where the current position can be checked and where the RNP/ANP can be checked, and the alert on the ND to alert the pilot to a navigation accuracy downgrade.

Note: "Monitoring" relates to FTE and NSE and it refers to the monitoring of the aircraft's performance, ability to determine positioning error (NSE) and/or to follow the desired path (PDE).

IMPORTANT NOTE: It seems that the Polish, UK and Portuguese CAAs have updated the correct option for this question to "FTE and NSE" - we believe that the remaining CAAs will follow. However, it is still possible that some authorities consider "FTE only" to be the correct option. Please let us know if you come across this question in your official exams.