Lena

Insurance policy and a property of the bar at the same time. Separating those two is what makes it choosable.

Superfat is two things at once, and separating them is what makes the number choosable.

It is an insurance policy. Your oils have a stated saponification value, your scale has a tolerance, and the lye takes up moisture from the air. Superfat means using slightly less lye than the arithmetic says, so that even with all those small errors stacked the wrong way, there is no free lye left in the finished bar. That is why almost nobody goes to zero, you would be betting on every measurement being perfect.

It is a property of the bar. Unsaponified oil left in the soap makes it feel less stripping on the skin, and contributes to a creamier lather.

So the trade at each end:

Low, around one to three percent. Harder, longer lasting bars with more cleaning power. This is why laundry and utility soaps sit down here. Less margin for error, so it wants accurate scales and fresh oil values.

Middle, around five percent. The default, and it is the default for good reason: enough margin to be safe, enough free oil to be pleasant.

High, eight to ten and above. Gentler feeling, softer bar, shorter life, and a real increase in the risk of the free oils going rancid over time. Above ten it starts trading away too much.

That rancidity point is the one people miss, and it connects directly to a problem you may already have seen.