What is a key feature of alert areas?

A key feature of alert areas is that they may contain unusual types of aerial activity. These zones are designated to inform pilots about potential hazards associated with specific flight operations, which may include activities such as parachuting, aerial photography, or military exercises. The intent is to promote safety by alerting pilots to the presence of these activities so they can navigate in a way that minimizes risk.

Unlike entirely restricted airspace, alert areas do not prohibit aircraft from flying through them; rather, they indicate that caution should be exercised. Therefore, pilots are encouraged to maintain situational awareness when operating near these areas to avoid potential conflicts with the unusual activities taking place. The other options do not accurately reflect the nature of alert areas, as they are not strictly off-limits, not exclusively controlled by the military, and can be manned or unmanned depending on the specific activities being conducted.