Why is the type of XIsInEffect Integer on the UNSAccountInUNSGroup table

Hi,

I have observed that the type of the XIsInEffect ususally Boolean, but on the UNSAccountInUNSGroup table it is specified as an Integer. Why is that the case? What could a value other than 1 or 0 indicate in terms of an assignment being in effect?

(Observed in an 8.1.3 and an 9.1.0 environment) 

Thanks in advance

John