Range Tribune HE : This tap on or off?

[janzihw]

just moved to UK. I have range tribune HE heating system.
1. What does the tap circled in red(attached image) do? Should it be on or off?
2. I have two power switches for this, one for regular and one for immersion. Due to the twisted wiring it is very difficult to follow the wires and see which switch is doing what. How do I identify which switch turns on immersion?

 

[terryplumb]

If water is coming out of all hot taps ( even if it's cold) you don't need to touch the stopcock circled in red.
Both the lower and higher immersion heaters will heat water ,the higher one will only heat water above ,( so won't heat the full cylinder),and the lower one will heat fully.
Sometimes the lower one will be electrically connected to a lower rate tariff ,and only work during the early hours around 11pm to 5 am roughly.