Best answer I could find
http://homepage.ntlworld.com/booty.w...TAR_decode.htm
EGZZ 231020Z 02006KT 4000 0900NE R27/0600U R32/0150D PRFG OVC007 12/11 Q1028
4000 .. 0900NE .. The first figure given is the 'prevailing visibility', which can be regarded as the 'best' figure that can be applied to at least 50% of the horizon (contiguously or otherwise). So, for example, if the visibility varies from 8km down to 4000m for at least half of the visible horizon, the prevailing visibility is 4000m. It is important to note that the visibility may be lower than this figure elsewhere, but for deviations to be reported, they must obey certain rules: if the visibility in one particular direction is less than 1500m or is less than half of the prevailing figure, then the lowest visibility observed (900m in above) is reported, with the direction shown (NE). If the lowest value applies in several directions, then the 'most operationally significant' direction is given. If the visibility is fluctuating wildly (e.g. rapid shower transistion), then only the lowest visibility is reported.
Where the observation is a fully-automated one (e.g. an 'AUTO' ob.), then no variation with direction of visibility is usually possible, and the letter-group NDV is appended to the visibility value.