The vis is forecast to be 800m from 2512 to 2518.
Outside that time period, it’s forecast to be 6000m.
There is no ‘precedence’ involved for the visibility figure, although it’s true that some TAFs do invoke that principle.
This is a good example of why I prefer the North American format, which gives ‘FM’ time periods, meaning ‘From’. I think it reduces the chance of misunderstanding as it is a full forecast from that particular time until the next ‘FM’ time period. No need to try to remember which element(s) of the previous period still apply.