I think all of those terms are Rock River Arms terminology.
The great thing about AR15s is that so many companies make them, and they are all pretty much intechangeable.
RRA, Colt, LMT, Bushmaster, Sabre, Stag, ....... many others I'm sure someone will chime in shortly with a few more brands I forgot.
My advice is to buy a complete factory rifle to begin with. As you get accustomed to the AR platform, and spend time here, you will change it over time, and probably wind up with several more as your wallet allows.
If you juat want a general purpose blaster/plinking rifle, get a 16 inch carbine (16 inch barrel with a carbine length gas system) or a 16 inch midlength (again, 16 inch barrel with a midlength gas system). If you want more of a target rifle, a 20 inch rifle will be fine. Think about what you want to do with it and you will get better advice.
It is a pretty modular system, so one lower can have several uppers. The lower reciever is technically "the gun" so once you buy that with the associated paperwork, you can order upper recievers all day long off of the internet and have them mailed to your home.
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