What is the property “SelectIs” created for?

It’s about a DiDesk applet. In the configuration for data selection <Ctrl+K> I create a SL-calculated column via right MT > New field. With right MT > properties I see the created expression as property “SelectExpression” and below it a second property “SelectIs”, which I can delete completely without disturbing the functionality.
I mean, the property that contains my expression is already recognizable as “Expression”. Why then the seemingly superfluous second property “SelektIs”?

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