Honestly one of the things I'm most excited for with ARMA Reforger and the eventual ARMA 4 is the game allowing terrain features that you just couldn't get in previous versions of ARMA/The Real Virtuality Engine, at least without some massive caveats. Things like narrow gullies along roadside, river/water streams, and even terraced terrain are just much more feasible with the Enfusion Engine.
By the nature of how AI navigation works in ARMA 3 and before, the AI can only reliably navigate on the height map ground of a given terrain as well as roads. Things like buildings or bridges, they needed to be given special nav nodes on them so they can navigate them point to point (it looks very robotic in action). These nodes don't exist on rocks, so an AI can't navigate across them even if it wanted to, and if you plopped an AI onto one, it would be stranded, only able to look around and shoot.
Now in ARMA Reforger/The Enfusion engine, they've introduced AI Nav meshes that cover the game world's terrain, both over terrain objects as well as over bridges and building interiors. This grants them overall much more flexibility in navigation and reduces the otherwise robotic behavior they exhibited in past games. On top of that (at least from what I've seen poking around in the game engine), there can be multiple types of AI nav meshes made, which may open up the possibility of certain corridors or paths opened for certain types of AI (vehicle, animal, civilian, etc).
Also, the AI nav meshes seem to take into consideration height information, which may play a role in visibility/spotting as well as tactical AI. Which is super exciting.
