Gini statistic in Interactive binning node

[Dan19]

Hello!
I am reading SAS documentation regarding  Interactive binning node. I saw the formula of Gini statistic (image attached). The Gini formula seems not to be standard one. So, it's interesting to me how this formula is derived, and is there a link to, e.g. an article, which explains this formula? Or can someone explain it?
Thanks!

[ballardw]

I suggest also providing a similar link to what you consider the "standard".

It is not impossible that the one in the SAS documentation is equivalent just different calculation order or expression.

Caveat: not any expertise in Gini but some experience with alternate calculation methods.

[Dan19]