There is no special setup mode. This feature is thought to be able to overwrite the default sxs or scomc parameters and make sxs or scomc start in log-mode (log to file), so that it becomes more clear why the specific module fails to start during setup. Setup always writes first the default sxs parameters. Then setup will look for a .reg file called "parameters.reg" in the setup location (same folder as the setup package).
This "parameters.reg" file will be provided by the Support department if there is a need for it. It is not present in the distribution package.
If the .reg file is present, setup imports the data specified in the file, so it overwrites the registry data of the parameters specified in the file. If the .reg file is not present, setup continues as usual (it only logs that there was no .reg file). Then sxs is started with the parameters written in the registry.
It is also possible to use this feature of specifying special sxs parameters in an unattended installation.