Help contains no information on how to configure the arguments to an external application for this option in the Preferences. The help file does show in its out-of-date example arguments such as ${firstFile} and ${secondFile}. Where are these file macros described? Are there other argument macros, eg, ${firstTitle}, etc?
when the working copy file is compared with a history revision,
when a conflict is edited
The parameters ${firstFile} and ${secondFile} specify the positions of the two compared files in the command line for the external diff application. These are the only parameters replaced by Syncro SVN Client in the command for the external application.