Loft water tank outputs

[marchcambs]

I have a gas fired central heating system with indirect hot water heating. The large tank in my loft has 3 outlets all with isolating taps.
The first is the cold input to the power shower.
The second is the input to the hot water tank.
The third does not seem to go anywhere. I have isolated it and nothing stops working.
Any ideas please?

[plumbbob]

An old second shower or a feed to a bidet? A feed to the bath cold that may or may not still be in use? They can also supply the toilet cistern.