I believe it has nothing to do with what we perceive to be a regional - aircraft type, pilot experience level, etc.
It is "US term for an air
carrier with annual
operating revenue below $100 million."
The U.S. Department of Transportation (DOT) has categorized airlines based on their annual revenues into three groups: major, national, and regional/commuter. Major airlines have more than $1 billion in annual revenues. This category once included Eastern, Pan Am, Northwest Airlines, Continental, Republic, America West, and Trans World Airlines (TWA). By the early 1990s, many of these companies were in some form of bankruptcy or had shut down operations. The result of these and other closings was the consolidation of assets among the three strongest majors: American Airlines, Delta, and United. New to this category was Southwest Airlines, formerly a national airline, which offered short-haul, point-to-point service with few amenities.
Airlines with annual revenue of $100 million to $1 billion are generally classified as national airlines. Although this category is called "national," the name is not based on geographic boundaries, as only a small number of carriers actually have nationwide routes.
A carrier with less than $100 million in annual revenue is classified as a regional/commuter airline. Some of the top regional carriers during the middle of the first decade of the 2000s were American Eagle, Sky West, Express Jet, and US Airways. While the major airlines struggled, regional carriers fared better during the first half of the 2000s, growing at about twice the rate of the national carriers. By 2011 the Regional Airline Association (RAA) reported that there were about 13,000 regional airline flights per day, and regional airlines were operating more than half of the nation's commercial schedule.
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