Right now, the "middle of the market" is the most dynamic battleground for longtime rivals Boeing (NYSE:BA) and Airbus (NASDAQOTH:EADSY). Boeing recently has been trailing badly, but it will begin its comeback effort this week by launching the new 737 MAX 10 at the Paris Air Show.

Unfortunately, there's a lot of confusion about what the middle of the market (MoM) opportunity is and how the various existing and potential Boeing and Airbus designs stack up against one another. Here's a brief primer on this key segment of the aircraft market.

Filling the gap

Just a few years ago, commercial aircraft purchases clustered in two quite distinct segments of the market. For short-haul routes, the Boeing 737-800 and Airbus A320 were the main options. Those two planes seat about 150-160 passengers in a typical two-class configuration and can fly about 2,500-3,000 miles under real-life conditions.

A Virgin America A320.

Planes with 150-160 seats are the highest-volume market segment. Image source: Virgin America.

Meanwhile, for their longer international routes, airlines mainly opted for large planes like Airbus' A330-300, Boeing's 787-9, and Boeing's 777-300ER. These planes hold 250-350, or more, passengers in a typical premium configuration, with full flat-bed business class seats, and can fly at least 6,000 miles -- and in the case of the Boeing jets, more than 8,000 miles.

However, airlines recently have become interested in planes that span the gap between the traditional narrow-body and wide-body segments in terms of size and range. This is the MoM segment. But aside from being in the middle, the MoM segment remains poorly defined.

How big is a MoM plane?

Thus far, Airbus has been the clear MoM leader, thanks to the soaring popularity of its A321 and A321neo jets. In fact, Airbus executives sometimes claim that they already have the MoM segment covered, because the A321neo has a maximum capacity of 240, while the A330-800neo has a typical capacity of 257.

Yet this is an apples-to-oranges comparison that glosses over dramatic differences in cabin density. The MoM segment really encompasses aircraft that can carry 150-250 passengers in a typical long-haul configuration, including flat-bed business-class seats.

The A330-800neo is at the very high end of this slice of the market in terms of capacity. The A321neo and its long-range sibling, the A321LR, are at the very low end. For example, JetBlue Airways outfits its transcontinental A321s with 159 seats. The carrier has hinted that, if it uses the A321LR for transatlantic flights, it would have even fewer seats.

Thus, even if the A321neo and A330-800neo can both be considered "MoM" planes, there's still a big gap between them in terms of capacity. That creates some white space for Boeing to enter the market.

How far should it fly?

Range is another key consideration for airlines. To be sure, the vast majority of airline routes are no more than 3,000 miles. The A321neo and Boeing's new 737 MAX 10 are well-suited to these shorter routes.

However, many airlines want a MoM-size plane with the range to fly some international routes. Over the past 15 years, Boeing's 757-200 has carved out a niche for itself flying transatlantic routes of up to 4,000 miles with about 170 seats. However, the 757 has been out of production for more than a decade and is not very fuel efficient.

In early 2015, Airbus launched the A321LR with specifications that roughly match the 757-200. The A321LR thus represents the first true MoM plane since the 757. It has already found favor with several transatlantic operators.

A rendering of a Norwegian Air A321LR.

Norwegian Air plans to fly the A321LR on transatlantic routes. Image source: Airbus.

That said, when airlines have tried to push the 757-200 to the edge of its range capabilities, they have found that they often need to make costly fuel stops when facing strong headwinds during the winter season. The A321LR will probably have similar limitations. Furthermore, an extra 500-1,000 miles of range would open up a ton of new route opportunities.

Thus, Boeing is strongly considering offering an all-new jet -- presumably called the 797 -- that could fly 225-260 passengers as much as 4,500-5,000 miles. This would slot in well above the A321LR in terms of both range and seating capacity.

From its current stable of planes, Airbus could offer the A330-800neo as a competitor. But the A330-800neo was built to be able to fly more than 7,000 miles. As a result, it weighs too much to have competitive unit costs relative to a hypothetical 797. Instead, Airbus may design an even bigger A320-family member -- the A322 -- with a new wing, offering higher seating capacity than the A321LR, and perhaps 4,500 miles of range.

What do airlines want?

Broadly construed, the MoM segment covers airplanes that can carry 150-250 passengers in a typical premium-heavy international configuration. Virtually every plane in this size range can fly at least 3,000 miles, and that's enough for many airlines. However, some airlines want to fly 4,000 miles or further with a plane of this size.

Airbus is well-positioned to dominate the lower end of this segment. With the A321LR and a potential A322, it would cover the 150-200 seat range with planes that could fly 4,000-5,000 miles. By contrast, Boeing seems more interested in the upper end of the segment: planes that can carry at least 200 seats in an international configuration and fly up to 5,000 miles.

Who wins will depend on what airlines want. If airlines are looking for a relatively cheap airplane that's a little more capable than what exists today, Airbus will be the winner. But if they want a plane that unlocks completely new opportunities, Boeing may have the advantage.