From SKYbrary Wiki
A danger area is an airspace of defined dimensions within which activities dangerous to the flight of aircraft may exist at specified times. (ICAO Annex 11: Air Traffic Services)
Most danger areas are operated by military authorities. The vertical and lateral dimensions of danger areas are publicised in national Aeronautical Information Publications (AIPs) together with the hours of operation where applicable.
Danger areas are established around areas where hazardous operations are likley to take place. These include, for example, military exercises involving live firing, parachute dropping, violent and unpredictable aircraft manoeuvres, or the use of unmanned aerial systems(UAS).
Normal practice is to include a buffer zone around the planned area of activity within the physical dimensions of the danger area. Usually, the danger area is monitored by the responsible authorities who cease operations if unauthorised penetration occurs. However, it is the responsibility of pilots to avoid penetration of danger areas.
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.