ER70s2 vs ER70s6 for TIG

I kind of had the impression that you used 70s6 for your run of the mill mild steel and 70s2 for things like 4130 tube.
Recently watching some video on YouTube a fellow was showing off his skills and welding up some mild steel welded square tube with ER70s2. I had just asked the manager over at the local Praxair store what the practical difference was, and he said basically what I said in the first sentence of this post.
So anyway, what is the real world general application of the two types of wire?
I know. I know. Go melt some wire and see what happens. LOL.
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