There is a real mathematical difference between (a) discounting the lye versus (b) adding additional fat. A lye discount (a) takes the saponification value into consideration. Superfat (b) is a given amount of extra fat based on a simple percentage of the total starting fat in the recipe.
The two are approximately the same if the saponification values for the fats are close to the average sap value for all fats in the recipe. If the sap values of the fats vary widely from the average sap value, then (a) and (b) will not be the same.
But we've gone hammer and tong on this topic in the past, and I've not been able to explain it well enough to make sense in writing. So I'll stop here and just say ... it's a difference that is pretty much ignored and one that even I don't worry too much about. The soap recipe calculators are set up to calculate a lye discount, regardless of whether they call it "superfat" or "lye discount."
The two are approximately the same if the saponification values for the fats are close to the average sap value for all fats in the recipe. If the sap values of the fats vary widely from the average sap value, then (a) and (b) will not be the same.
But we've gone hammer and tong on this topic in the past, and I've not been able to explain it well enough to make sense in writing. So I'll stop here and just say ... it's a difference that is pretty much ignored and one that even I don't worry too much about. The soap recipe calculators are set up to calculate a lye discount, regardless of whether they call it "superfat" or "lye discount."