It's the "exterior perimeter".
I'd interpret that to be something like perimeter of the bounding box (if we're talking about rectangular robots). If the shape is more complex, I would wonder whether the intent is for the rule to apply to the convex hull shape that encloses the robot, or if there's some other criterion for defining what's an exterior feature, and what's an interior feature (and not included in the perimeter). I'd be inclined to accept any good-faith attempt that satisfies the 2/3 stipulation by some obvious metric.