Use this field to specify a variable whose values can, either singly or in combination, provide a unique identifier for each row. This variable is used to generate names for the distance variables and is copied to the output data set.The maximum length of the combined Prefix + ID variables is 32 characters minus the number of rows in the data set. If the form of the input data is already a distance matrix , this ID variable is used to label the cluster Dendrogram .All of the variables in the input data set are displayed in the Available Variables field.
Click to add the highlighted variable to the ID Variable field, as shown below: