The /ComparisonKeys switch(/ComparisonKeys:<table or view name as regular expression>:<index name>) allows you to specify a unique index to be used to identify rows for comparison. With /ComparisonKeys you can only specify an index as the comparison key, no other columns can be specified.
To specify a comparison key that is is not an index, use the GUI to set up and save a project with the settings you require. You can then use that project from the command line with the /Project switch.
https://documentation.red-gate.com/disp ... mmand+line