I came up with the idea, as far as I'm aware (please let me know if I'm in error!) I think the Soapee guy took the idea from our discussions here on SMF and ran with it. It's not on any other soap recipe calc that I'm aware of.
The number is the sum of the % stearic acid and % palmitic acid in the recipe. You can calculate it as Carolyn explained or you can look at the fatty acid profile of your recipe and add stearic and palmitic acids. Either way works. I have about the same expectations as Carolyn -- I'm looking for 25-30% for the combined palmitic and stearic when roughing out a typical bath soap recipe. Too low and the soap doesn't last sufficiently long in the shower. Too high and the soap doesn't lather as well as I'd like.
Stearic soap and palmitic soap are not very soluble in water, and these soaps are also add physical hardness (as in hard like a rock). These two properties contribute to the longevity of the soap, meaning how long the bar will last in the shower. It has nothing to do with volcanoes or overall shelf life or anything like that.
Other ingredients in the recipe can affect the longevity of the soap, so I use this number (and the other numbers for cleansing, bubbly, etc.) as a guideline only. Many beginning soapers obsess over formulating the perfect soap recipe and want to know what "numbers" they should should shoot for to create that Holy Grail recipe. There is no such thing. There are recipes that work well for most people most of the time, but no one recipe will ever be perfect.