What structure operates under the Chief of Staff, U.S. Air Force, but does not fit into any major command?

The structure that operates under the Chief of Staff, U.S. Air Force, and does not fit into any major command is the Direct Reporting Unit. These units are unique because they function independently of the major commands (MAJCOMs) yet still report directly to the Chief of Staff. This allows for a more focused management of specialized missions or activities that may not align neatly with the broader responsibilities of the MAJCOMs.

Direct Reporting Units often have specific operational or administrative functions, such as providing support capabilities that are essential across the Air Force but are not tied to the traditional command structure. This independence from MAJCOMs enables them to address unique challenges and provide targeted support more efficiently.

Field Operating Agencies, on the other hand, typically fall under a major command and are designed to provide operational or functional support to that command. Operational Wings are also aligned under major commands, fulfilling operational readiness and mission execution roles. Combined Commands, often seen in joint operations, involve integration between multiple military branches and typically do not report directly to the Chief of Staff in the same way that Direct Reporting Units do. Thus, the distinctive nature of Direct Reporting Units sets them apart as the correct answer in this context.