Exactly what makes a MOPAR a "Matching Numbers" car?

When I was a kid, I was on top of the world driving my 1971/1972 Demon (avitar). That's why I bought my project when I retired. I didn't care about numbers then, and I still don't as long as it's legal. But it sure is tough avoiding the rabbit hole when you are putting them back together......and you have a little cash!