Switch this option on if your current unit settings specify that you are using a period as the decimal separator, but you want to use a comma for constants in variable expressions.
If you have set this option and it causes a mismatch with your current settings this will be highlighted at the bottom of the Variables tab:
Up until variables are created, the mode is set automatically. For example, switching locales using Windows (with Impact running) will automatically apply those options as long as no variables have been created.
Once variables have been created, then you can only switch the options using the Options menu on the Variables Explorer. Doing so will confirm you want to make that change, and may then require you to update any expressions to switch between the old and new options. For example, changing period '.' to comma ',' in expressions or dimension expressions.
Updating expressions is NOT done automatically, it is purely a manual process. In addition, if you manually change any of the expression options before creating any variables then the automatic updating of settings is also disabled.