retainClassesByInd retain observations corresponding to
  specified class indexes
 
    subDataSet = dataSet.retainClassesByInd(classInds) returns
    a data set subDataSet containing only the observations that
    have targets with values that corresped to the specified
    class indexes.

Help for prtDataSetClass/retainClassesByInd is inherited from superclass prtDataInterfaceCategoricalTargets