Okay - BackPacker is quite right. This was what I missed from the FAA AC 90-96 Appendix 2:
The following system functions are the minimum required to conduct BRNAV/RNP-5 operations:
1. Continuous indication of aircraft position relative to track to be displayed to the pilot flying on a navigation display situated in his primary field of view;
NOTE: In addition, where the aircraft type certificate requires more than one
pilot, information to verify aircraft position must be displayed in the non-flying
pilot’s primary field of view.
2. Display of distance and bearing to the active (To) waypoint;
3. Display of ground speed or time to the active (To) waypoint;
4. Storage of waypoints; minimum of 4; and
5. Appropriate failure indication of the RNAV system, including the sensors.
Now I understand peterh337's point. Thank you.