The generational monikers bestowed on fighter jets aren’t quite as definitive as the people arguing in the comments below most articles tend to think. The corporations that design and build these jets, the military forces they’re operated by, and the analysts who spend their days pouring over comparisons and tech schematics may all provide you with a slightly different version of what it really means to be a fifth-generation fighter — and for good reason. There just isn’t any overarching governing body with the authority to tell American, Chinese, Russian and other industries what to call their planes.

Despite a lack of formal generational requirements, you’ll often find the industry has a way of balancing itself through the cost of available technology. Fifth-generation fighters, for instance, are widely accepted as aircraft that were designed with stealth intrinsic to the air frame, boasting advanced networking capabilities, and having efficient enough engines to “super cruise,” or maintain supersonic speeds without keeping their afterburners engaged. Technically, there are other qualifications, too, but neither Russia nor China’s “fifth generation” fighters currently meet even that third basic criteria, and yet their governments, the media, and most of the world still call them “fifth generation” fighters.

The point is, what makes for a new generation of fighters isn’t always that easy to pin down in lofty conversations about the future — but that’s precisely what the U.S. Air Force is currently working on with its Penetrating Counter Air (PCA) program.

The PCA program is the Air Force‘s plan to begin fielding a sixth-generation air superiority fighter sometime in the 2030s. For those who may be hazy as to what differentiates an air-superiority fighter from the multi-role qualifier usually adorning the F-35, the concept is simple enough: an air superiority fighter is purpose-built to engage with enemy aircraft. Multi-role fighters, on the other hand, often compromise acrobatics or speed in favor of absorbing other mission requirements, like close air support. The U.S. Air Force currently employs the world’s first fifth generation fighters (F-22 Raptor) and a sizable fleet of older but faster F-15s to fill this role.