I think I've got it broken down to a science Fame wise (Well at least how I see it) lol:
Mainstream: Common household name. Everyone recognizes the artist either from TV or always hearing their song on the radio. In other words, alot of white people knows who the artist is lol. Example: Lil Wayne, Rick Ross, 2 Chainz, Wiz Khalifa, etc.
Low Mainstream: Barely common name, only known to MOST for one song/album. People will forget/the hype will die down about the artist for awhile until their next project is being promoted. Example: J.Cole, Kendrick Lamar, Big Sean, Wale, etc. This category could also include 1-hit wonders.
Mainstream Underground: That zone where the artist is known by all Non-Radio Exclusive listeners. Possibly Indie. Most only care about staying true to their fans. Example: Tech N9ne (Perfect example,) Curren$y, Lupe, Blu, Joey Badass, Lil B, etc.
Another perfect example of this category is 08-2010 Wiz. Wiz was the "Underground King" before Black and Yellow pushed him over.
Then there comes Underground, Local, and then Off the Radar.
But like Rice said, it depends on how you see it. Most "Mainstream" artists have no control of their music or only care about appealing to the radio. They treat it like a popularity contest. An artist "Sells Out" when it clearly shows that they are solely focusing on satisfying their "Hit Record" fans instead of their loyal fans that got them famous in the first place. Again, Wiz is a perfect example. He wanted to make it seem like he still cared with Taylor Allderdice, but its obvious he doesn't with the stuff he points out now.