On a sectional chart, what does a blue circle with an 'R' inside represent?

3-4-3. Restricted Areas
a. Restricted areas contain airspace identified by an area on the surface of the earth within which the flight
of aircraft, while not wholly prohibited, is subject to restrictions. Activities within these areas must be confined
because of their nature or limitations imposed upon aircraft operations that are not a part of those activities or
both. Restricted areas denote the existence of unusual, often invisible, hazards to aircraft such as artillery firing,
aerial gunnery, or guided missiles. Penetration of restricted areas without authorization from the using or
controlling agency may be extremely hazardous to the aircraft and its occupants. Restricted areas are published
in the Federal Register and constitute 14 CFR part 73.
b. ATC facilities apply the following procedures when aircraft are operating on an IFR clearance (including
those cleared by ATC to maintain VFR-on-top) via a route which lies within joint-use restricted airspace.
1. If the restricted area is not active and has been released to the controlling agency (FAA), the ATC facility
will allow the aircraft to operate in the restricted airspace without issuing specific clearance for it to do so.
2. If the restricted area is active and has not been released to the controlling agency (FAA), the ATC facility
will issue a clearance which will ensure the aircraft avoids the restricted airspace unless it is on an approved
altitude reservation mission or has obtained its own permission to operate in the airspace and so informs the
controlling facility.
NOTE-
The above apply only to joint-use restricted airspace and not to prohibited and nonjoint-use airspace. For the latter
Special Use Airspace 3-4-1

AIM 2/20/25
categories, the ATC facility will issue a clearance so the aircraft will avoid the restricted airspace unless it is on an approved
altitude reservation mission or has obtained its own permission to operate in the airspace and so informs the controlling
facility.
c. Permanent restricted areas are charted on Sectional Aeronautical, VFR Terminal Area, and the appropriate
En Route charts.
NOTE-
Temporary restricted areas are not charted.