What is the San Francisco Font feature "Calculator"

I am looking through the output of calling CTFontCopyFeatures for the System font on iOS 11. I am familiar with most of the features that this output describes, however there is one that I haven't determined:

{
        CTFeatureTypeIdentifier = 35;
        CTFeatureTypeName = "Alternative Stylistic Sets";
        CTFeatureTypeSelectors =
        (
           {
                CTFeatureSelectorIdentifier = 18;
                CTFeatureSelectorName = Calculator;
            }
        );
 }

I tried enabling this feature but I haven't seen any change to the output in my experimentation. I tried showing a label of text with alphanumeric characters and haven't seen any difference.

I also haven't seen any Apple documentation or any hits Googling this feature.

Does anyone know what this feature does and how to use it correctly?

Thanks.

Doug Hill