to maintain an actual navigation performance (estimate of position uncertainty) of less than 5 and require alerting to when you are not able to maintain that performance
That is the case for basic 5.0 RNAV , but Eurocontrol is active in pushing
1.0 P-RNAV initially on high demand routes, obviously airlines will only be able operate certified compliant airplanes with this capability. 0.3 P-NAV is also around the corner.