I guess you would need to ask that question to the Batman Developers.
This “modified” could be anything. As soon as they extended or changed a base class, it is modified.
That can be anything, like you already said (graphics, ai, etc.). They could also used plugins to modify
and add something to the Engine.
The better question would be: What do you want to recreate from the Batman game? Because it seems to be
important for you to know what they have changed and if you need to do the same thing for your project.