What is the difference between SUA (Special Use Airspace) and FUA (Flexible Use Airspace)? The EUROCONTROL Concept of the Flexible Use of Airspace (FUA) is that:


  
  an ASM concept currently applied in the ECAC area and based on the fundamental principle that airspace should no longer be designated as
  either pure civil or military airspace, but rather be considered as
  one continuum in which all airspace user requirements have to be
  accommodated
  an airspace management concept applied in the European Civil Aviation Conference area on the basis of the “Airspace management
  handbook for the application of the concept of the flexible use of
  airspace” issued by Eurocontrol
  


EUROCONTROL also defines Special Use Airspace (SUA) as:


  Airspace of defined dimensions identified by an area on the surface of
  the earth wherein activities must be confined because of their nature
  and/or wherein limitations may be imposed upon aircraft operations
  that are not a part of those activities.


Question: what is the relationship exists between SUA and FUA? Can they be considered as the same thing? Is SUA a special type of FUA or vice-versa?
What is the difference essentially?
Flexible Use of Airspace is a concept rather than a part of airspace (which SUA is), where the whole airspace is used for both restricted and unrestricted (civil) activities. According to FAA:

• The FAA version of Flexible Use of Airspace (FUA) is called “Joint Use Airspace”
• Although not defined as such, the domestic United States could be labeled as being completely FUA airspace.

The Special Use Airspace (SUA) is a portion of the Flexible Use Airspace (FUA), in which unrelated activities have been restricted:

In accordance with the FUA concept, Special Use Airspace is a defined portion of airspace which may be temporarily reserved for specific activities which require the restriction or segregation of other traffic.

Basically, in a FUA, SUA will be a temporary restriction (rather than a permanent one). Eurocontrol also says the same thing:

The EUROCONTROL Concept of the Flexible Use of Airspace (FUA) is that:
Airspace is no longer designated as purely &quot;civil&quot; or &quot;military&quot; airspace, but considered as one continuum and allocated according to user requirements.
Any necessary airspace segregation is temporary, based on real-time usage within a specific time period.

Related concept is Advanced Flexible Use of Airspace in which SUA is allotted in real time.

An airspace management concept in which airspace is managed as a single entity and in which there are no fixed structures and airspace reservations for special airspace activity are allocated in real time.