Hey,
Chunked terrain is set when you go to map properties and apply mesh mode.
What ever number you set determines the size of the chunks I recall but then when this first appeared, we were to have added a line saying something like chhunked = 2; or something.
This was suppossed to have set any large terrain to cut up units for frustrum viewing so that the rest of it could be clipped out while player wasnt seeing them.
Anyway, it may help or set someone elses memory off ....