According to PANS-OPS (volume 1, 126.96.36.199): "In order to ensure acceptable clearance above obstacles during the departure phase, instrument departure procedures may be published as specific routes to be followed or as omnidirectional departures, together with procedure design gradients and details of significant obstacles."
The NavigationArea feature supports the encoding of sectors having minimum gradients and/or minimum altitudes, in relation with omnidirectional departures. The following diagram indicates the main elements:
For each NavigationAreaSector it is possible to specify one of more SectorDesign sets of parameters, each including a designGradient, a terminationAltitude and information about turn permissions. In the current model, NavigationArea is associated with a StandardInstrumentDeparture, through which it may be further associated with particular LandingTakeOffAreaCollection (one or more RunwayDirection and/or TouchDownLiftOff areas).