The RS is the V6. LS and LT are option packages, go build a Cobalt on the Chevy website and I think you will understand.
Per Road and Track, the L99 is the version of the LS3 that will come with the auto and has AFM, etc. That is consistent with what we heard before, that the auto and the manual would have different HP numbers. And current AFM does not work with a manual, so it fits.
"Those who choose the Tremec 6060 manual will get the LS3 from the Corvette, while buyers opting for the 6-speed automatic will get the L99 V-8 with active fuel management, which cycles back and forth between four and eight cylinders to boost fuel economy. The LS3 is expected to make about 420 bhp and 408 lb.-ft. of torque, while the L99 is estimated at 395 bhp and 395 lb.-ft. of torque. The V-6 will run on regular unleaded gasoline, while both V-8s are specified to take premium."
This was noted above Shadowsong above, this is the details they posted.