Alan - I guess what I mean by "residual sugar" is any fermentable sugar in a wine, after the initial fermentation has stopped. It doesn't really matter to the yeast whether the sugar is there because fermentation stopped a bit early for some reason, or because you added something after the initial fermentation completed. If there is fermentable sugar in the wine (and you have't fortified it with spirits up to, say, 18% or more alcohol), then there is a risk of renewed fermentation. If you don't want renewed fermentation (you want it if you're making sparkling wine, like Champagne, but not otherwise) then you need to do something to prevent fermentation from starting up again. Commercial winemakers can use filters that remove all the yeast cells, and bottle in a sufficiently sterile environment that the risk is minimal. For home winemakers, that approach normally isn't feasible, so the simplest and easiest approach is to use sorbate.
I don't often sweeten my wines, so I don't use the commercial wine sweetening or "wine conditioner" products. I gather that some of them contain sorbates as well as sugar or glycerin or whatever. In those instances where I do want to sweeten a wine, I want to make sure there is enough sorbate, so I add that separately, then add table sugar (slowly) to taste. The commercial products may work well in some situations; my concern about relying solely on the sorbate included with a "wine sweetener" product is that adding just a small amount to the wine might result in a sorbate level that is too low to be effective.
Doug
