Setup Comparer is a small application that can display two setups side by side for comparison.
Setup Comparer is an .HTA file. That's a Microsoft HTML file standard. It opens in an Explorer windows under Microsoft HTML Application host. Since it does not write anything anywhere, it does not need any permissions, it's a harmless application. There is no executable file, it can only read the setup file you point it to.
Setup Comparer does not have the capability to edit the setup files, it's only capable of opening two setups and display them all neatly formatted, in Metric or Imperial units.
As opposed to starting Steam, then starting the sim just to check some settings. It's a lot quicker with Setup Comparer.