In the Pilots Handbook of Aeronautical Knowledge they discuss the different types of airspace and briefly mention numbered Alert Areas. I don’t remember ever seeing one, so I looked on the charts to find one. I also stumbled across some other areas that the chart makers want pilots to be aware of. Some of these are used when a specific area is not demarcated e.g. caution and warning. While some have clearly demarcated areas and altitudes e.g. National Security Notice Areas.
Alert areas are depicted on aeronautical charts with an “A” followed by a number (e.g., A-211) to inform nonparticipating pilots of areas that may contain a high volume of pilot training or an unusual type of aerial activity. Pilots should exercise caution in alert areas. All activity within an alert area shall be conducted in accordance with regulations, without waiver, and pilots of participating aircraft, as well as pilots transiting the area, shall be equally responsible for collision avoidance.
Alert Area A-292 is prominently labeled in magenta on the chart and alerts pilots to a high volume of traffic in the MOA. There is another area outside the MOA (labeled in blue) that pilots also need to be aware of.
From looking at the charts, it appears that alerts, warnings, and notices within MOAs are depicted in magenta—just like the MOAs. Restricted Areas are indicated on the charts in blue and alerts, warnings, and notices within Restricted Areas are depicted in blue. Outside of MOAs and Restricted Areas they are depicted in blue. Caution Areas can appear in magenta or blue.
In this area, there is a National Security Notice Area in magenta where pilots are requested to remain above 8000′ around the ordnance depot and an area where pilots are cautioned that there is a high volume of flight training. At the upper right, there are also symbols for ultralight and glider activity. These appear to be always labeled in magenta.
This chart shows a rather large area (the Hanford Nuclear Site) where pilots are requested to remain above 1800′ for Reasons of National Security.
Wildlife Areas are also marked on the charts in blue with a Notice to Pilots advising of the minimum requested altitude.