--contains only the "right-handed" isomer of alpha lipoic molecules, which is the only one the body uses. Many organic molecules come in both "right-handed and "left-handed" forms, based on the bend angles in the molecular bonds, but the body can only process certain molecular shapes.

Alpha Lipoic contains both isomers, so a certain amount of each dose is wasted--passed through without being biologically active.

What this means is that one can use far less R-lipoic than alpha lipoic for a similar effect--a ration of 1:6 to 1:10 is often cited. This means people who have acidity problems with alpha lipoic can often avoid the worst of that by taking smaller doses of R-lipoic.

See:

http://en.wikipedia.org/wiki/Lipoic_acid