You can save results for an External element by clicking Final Values and/or Time Histories. Checking one of these causes all output arguments to be saved. If the element has multiple outputs, and you wish to save only one or two of these as results, you can use the context menu of the output (accessed via a right-click) in the browser to turn a particular output on or off. In this case, the checkbox in the dialog will become gray (indicating that some of the results will be saved). The display units for these outputs are defined when the output is created.
Note: An External element does not have a primary output.