RAD - Route Availability Document
The Route Availability Document is a flight-planning document, which integrates both structural and ATFCM requirements, geographically and vertically, following an appropriate collaborative decision making process between the [network manager][nm] and the operational stakeholders. It is a ‘central’ source of airspace utilisation rules and availability defined and communicated by the ANSPs, 56 days before the start of the concerned Aeronautical Information Regulation and Control (AIRAC) cycle.
It facilitates flight planning in order to improve Air traffic Flow and Capacity Management while allowing aircraft operators’ flight planning flexibility.
It also enables ANSPs to maximise capacity and reduce complexity by defining restrictions that prevent disruption to the organised system of major traffic flows through the congested areas with due regard to aircraft operators requirements.
Airspace utilisation page at EUROCONTROL website.