All airlines operates 121 even regionals, Pinnacle operates 121. There are exceptions to the rule but any operation with multiple 50 seat RJ, or bigger, will most likely be operating under 121. Also I think UPS, and Fedex both operate under 121 rules as well. Another aspect is the size of the aircraft an cargo/passengers carried. Operating under 135 you can only carry so much cargo/passengers. Ameriflight is a cargo operation they can't even fully load some of there aircraft because of this weight restriction. That would be a simple way of looking at it; apologies for the grammar