Use this field to specify variables that are to be retained in the output data set. They are often the same as one or more of the By Variables specified previously.All of the variables in the specified input data set are displayed in the Available Variables field.
Highlight the variables to be used to key the merge. To highlight multiple variables at once, hold down while left-clicking on them.
Click to add the highlighted variable(s) to the Variables to Keep in Output field, as shown below: