When you make soap, you can use exactly the right amount of lye to exactly react with the fat. This called zero "superfat" or zero "lye discount". Which term you use depends on your point of view, but for the sake of keeping it simple, the two terms mean basically the same thing.
Rather than use zero superfat, you can use a little MORE lye than is strictly needed. This would be called is a "negative" superfat or negative lye discount. This lye heavy soap will be unsafe for use on skin. About the only time a negative superfat is used might be when a person is making a household or laundry soap, but that's not very common. It is more common for people to have a lye heavy soap from mistakes made when designing the recipe or when making the soap.
Another alternative is to use a little LESS lye than is strictly needed or a little MORE fat than is strictly needed. This means the soap is fat heavy -- some fat will be left unsaponified in the soap. This "positive" superfat or lye discount does two things --
First, it adds a margin of safety to ensure your soap does not contain excess lye. This is why the soap recipe calculators have a default positive superfat or lye discount built in -- the usual I've seen is 5%. All of the calculations needed for a soap recipe are based on estimates of the purity of the lye and estimated saponification values for the fats. Because they're all estimates, however good, it's always best to err on the side of slightly too much fat for safety.
The second reason for a positive superfat or lye discount is that a bit of excess fat can also modify the qualities of the soap to make it less cleansing or milder yet to the skin.
There's more to this, but this is a beginning..... Hope it helps.
Superfating is also referred to as lye discount. Standard amount is 5 percent. You lye calculator will automatically lower your lye so that 5 percent of your oils do not saponify. This is a safety buffer to assure no lyre will be left in the soap. You don't need to do anything special with Superfating just follow the measurements given by you lye calculator.
Super fatting is when you have a little unsaponified oil in your soap . most soap calculators will factor this in for you , all you have to do is enter the percentage you would like to have. if you search through this site archives you find similar thread on the same subject. it should be noted that what most soap calculators do is called " lye discounting " but in the end gives you the same desired results. true to term superfatting is done in HP where the desired oil / fat is added at the end of the cook . CP is lye discounting .
E.T.A : i see two of my fellow members beat me to the punch but you get the meaning and science of it all
Another definition you will see applies to hot process. When I first started soaping 10+ years ago, the "rule" was that you had a special oil at usually 5% - something like sweet almond, avocado, etc - and you added that 5% after your soap hit trace, right before you poured. That's been proven to be pointless - the lye takes what it wants. No point in holding back your "special" oil in CP.
But it can actually be beneficial in HP. HP = Hot Process. That means you will mix up your lye water and oils, minus your special oil. Then you will cook the soap (Google if you'd like to see pics) and at the end of the cook, you add fragrance and your special oil. This is often called the superfatting oil. (We need a new word.) After cooking the soap for a while, most of the lye has been used up so the special oil will remain unsaponified, for the most part.
The definition previously explained is the one we mean 95% of the time.