The single combination window is used to calculate the totals and exceeds for a single combination of specified racks. This is different from a full run in that only one combination is run, and it's not randomly selected. This is useful for quick questions about resource usage.
When you call the single combination window up, you see this window where you can select which racks you want to have in your combination. You can also select which run's parameters you want to use (this defines the capabilities the totals are compared to, and the topology for the combination, and things like that) and whether or not this combination requires SAMS.
On clicking "run", you are shown this window. It prints the exact command it ran to do the combination, and the results. The results are in the form "field_name=value", just like they would be in the output table. You can copy and paste from this window to another document if you want to save these results.
Running a single combination should not take longer than a couple seconds; if it does then something is wrong.