https://www.google.com/amp/www.chicagotribune.com/sports/college/ct-northern-illinois-stuns-nebraska-niu-20170916-story,amp.html
It's a status. They use it to categorize the Independent programs that have the accoutrements and history of a P5.
There were a lot of them until the early 1990's. Miami, Penn State, Boston College, Florida State, Pitt, Syracuse. Today, Notre Dame and BYU are the only indys I've seen categorized as "Major."
None of the other schools that have been (or will be) Indy over the last 20 years have been called major that I can find. Army, Navy, UMass, Idaho, UConn, New Mexico State, Liberty, Utah State, Central Florida, South Florida, Temple etc.