What is the difference between SUA (Special Use Airspace) and FUA (Flexible Use Airspace)?
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.