I am also an RSD sufferer with both a cervical and lumbar SCS. As Rae stated, Burntmarshmallow's explaination is accurate. A couple things I'd like to add is that the SCS covers a much larger area than the TENS is able to do. Also, as the TENS covers more of the skins surface, rather the SCS more internal, you know how a flare up with this monster can make the surface of your skin feel like it's on fire??? Well, as much as I LOVE both my SCSs, it does NOT provide total relief when your skin feels like it's aflame. My ANS Rep told me because the SCS works from the inside out. Perhaps when I get that ol' flaming feeling I should pull out the dusty TENS unit

I hope that helps!! Vanna