Indicates that the value, or some other quantifiable property, of this AtkObject cannot be fully determined.
In the case of a large data set in which the total number of items in that set is unknown (e.g. 1 of 999+), implementors should expose the currently-known set size (999) along with this state. In the case of a check box, this state should be used to indicate that the check box is a tri-state check box which is currently neither checked nor unchecked.