I don't have the game yet and was wondering what can be done with this editor? When you say it allows us to edit campaign and scenario scripts it sounds like you're referring to AI scripts, or is it something else?
Can it be used to revise units, unit placement, etc. in saved games, or does all of that have to be done in an xml file?
you can edit units, but only to a certain degree. You cannot change their basic values, but add support units, increase experience etc.
You can trigger events, like if brigde A is not occupied by the Germans in turn 4, it will be blown, making it impossible for tanks to cross a river.
Or you can have reinforcements enter a scenario on a certain action or turn. E.g. the Germans cross a river and the Stavka releases a combat group as a reaction - this can be seen in the Smolensk scenario.
There are plenty such things that can be done with the combination of actions.
You can completely edit existing maps and their values this way, make translations as the editor supports German and English (that could be expanded to other languages, if the game supports it) and tons of other stuff.
The editor also supports campaign values, so those can the done there, too.
Here is a screenshot of the editor: