Arma maps sometimes use water, but others have endless procedural wastelands beyond the edge of the map. It's pretty obvious where the line is, since ground clutter like trees and bushes cut off, and all roads end, but nothing prevents you from driving or flying forever, and fights between aircraft often range far beyond the map boundaries. I can remember a few times when I'd win a dogfight, but run out of fuel and have to ditch my plane 20km or more outside the map, which made for some awkward conversations with the rescue chopper crew. "Where exactly are you down again?"
Basically, the terrain is endless, but incredibly boring, so people stick within the gamespace.