As you probably know, InterSystems IRIS has a mechanism for auto-generating methods. So, when you create an index in a class, you create methods that make your work easier. There is also a good article on Community that describes such methods.
Take, for example, IndexNameOpen(val), where IndexName is the name of the corresponding index. This method returns an object in which the value of this index corresponds to the value of val.
But this method is available only for unique indexes.