You mean the visibility mod?
Well, like the name says...8k gives you a horizon and visual limit of 8000 metres, while 16 k sets it around 16000.
Advantages of having it at 16k: 1) More physically realistic, 2) Improved gameplay since you can locate and track enemies from greater distances, 3) Looks better.
Drawbacks: 1) Vampire crew at night: To have them see so far as 16000 metres by day results in irrealistically far spotting distances by night, 2) Framerates drop, specially near land or heavy convoys.
More or less that sums it up. I personally use the 16K, as included in NYGM
