Underground is not a specific genre of rap or any music. It's basically the people that haven't made it to the top yet. UNDERGROUND
People get this confused because you here mostly back packin rap music dubbed as underground. That's because the mainstream of people are not feelin the back-packin shit. So the BP-Mc's dwell with an underground "following".
Cash money & No limit was underground for a long time, sellin shit out the trunks, and they were very famous in New Orleans, but it was until they got big distrubution deals that they were able to reach everybody, and broke into the mainstream.

Commercial just means your work is product now. The lyrics aren't hardcore because only a small percentage of people understand "molecular structurs that'll rattle and rupture the universe spacetime algara-rhythms". They just want to be entertained, they just wanna have fun singin along with you.

You get paid to rhyme, and to ME, that is the highest form of respect and flattery. To actually have someone like your shit soooo much, that they make an effort to go out and buy it with their hard earned money.

Fuck a bunch of funky ass dudes talkin about "yeah keep it real son, you maaaad nice". I can't do nothin with that!!