--R lipoic or alpha lipoic acid; the latter is simply a refinement that has come along fairly recently that allows for somewhat smalelr dosing.

Without getting into too much biochemistry, many molecules in the body, and many nutrient molecules, have structures that can be described as "right-handed" or "left-handed" depending on how the bonds among the atoms are configured (the angles at which they join). Alpha lipoic acid consists of both configurations of alpha lipoic molecules, but it turns out our bodies can only use the right-handed ones, so R-lipoic was developed to consist of only right-handed lipoic molecules.

What this does is enable a given dose of R-lipoic to have a much greater percentage of usable molecules, such that one can take a much smaller dose to get a similar effect to alpha-lipoic. Often, a 6:1 to 10:1 ration is cited; one can take 60-100mg/day of R-lipoic to get a similar effect to 600mg/day of alpha lipoic, which is a common daily dosing for that. Since some people report reflux with higher doses of lipoic--it is, after all an acid--being able to take a smaller dosage can be helpful (as can taking the supplement with food).