It's the amount of BSD you have from gross BSD after paying all the technical debts.

But then what does that make FreeBSD?

EBITDABSD.

It's a BSD variant dedicated to running on a wide variety of hardware.

One of the running jokes is that you can "run it on a toaster" — see https://news.ycombinator.com/item?id=45712368

If it can run DooM it can run NetBSD.

Not really. Doom just requires a 32 bit CPU (maybe someone even kluged it together on a 16 bit platform?) but NetBSD requires a CPU with an MMU.

NetBSD is the slim, small, traditional BSD that has an emphasis on clean code and portability. It's great for small jobs and it'll run on that old SPARC that's collecting dust in the closet. It's simpler than FreeBSD (the industrial strength BSD) and doesn't have the hyper focus on security that OpenBSD does.