From SKYbrary Wiki
A restricted area is an airspace of defined dimensions above the land areas or territorial waters of a State, within which the flight of aircraft is restricted in accordance with specific conditions. (ICAO Annex 2: Rules of the Air)
The vertical and lateral dimensions of restricted areas are publicised in national Aeronautical Information Publications (AIPs) together with the hours of operation where applicable.
Historically, many Danger Areas and restricted areas have been inactive for much of the time for which they have been notified as being active. This has led to the development of the Flexible Use of Airspace (FUA) Concept to optimise the use of airspace within Europe.
With the application of the FUA Concept, airspace is no longer designated as "civil" or "military" airspace, but considered as one continuum and allocated according to user requirements.
The FUA Concept allows the maximum shared use of airspace through enhanced civil/military co-ordination. The application of the FUA Concept ensures that any airspace segregation is temporary and based on real use for a specified time period.