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.