First of all, welcome!
NOM stands for Not Original Motor. Perhaps it is a correct type replacement or even a correct warranty replacement engine, but it isn't the one it left the factory with. That's all.
It is significant in the case of a car with a rare high-performance engine. When the original engine (with serial number matching the car) is gone, then whose to say that someone somewhere along the way has simply upgraded the car with a desirable engine in an effort to raise the value of the car. Now, a nice high-performance engine that isn't original to the car, who’s to say that doesn't make the car cool? Still, there is significant value attached to a car with the original engine when it is a rare, high-performance version. People read into it as being a virgin that has never been abused or blown up. Usually it is the high-performance versions that are treated to abuse, thus the survival rates are low. The rarity of that combined with low production and the current day desirability place a high premium on the original engine cars. Somewhere along the way things got weird, people started trying to fake engine stamps in an effort to boost the value and the real value of appreciating the machinery took a back seat. It started being more about the dollars that a car would bring than what the car really stood for; the attitude and the feeling. Like it was once said, people don’t make fake coal, they make fake diamonds. With all the silliness you can see how some people just don’t want anything to do with numbers-matching, and who could blame them? When correct dipsticks are suddenly priced out of sight, yet some pays the price, some real enthusiasts just shake their head and walk away from that aspect of the hobby. Still, there really is something significant about a car that has survived against the odds, with rare and desirable equipment. That definitely does deserve recognition.