What type of valve is this?

[Fingersgreen]

This valve is fitted to the return pipe from the HW cylinder, what is it for and why does the pipe after the valve split two ways?

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I'm simply interested, nothing more

 

[Steelmasons]

Called a lockshield valve (anti tamper) which is installed to reduce flow rate through the cylinder coil in order to eliminate short circulation through the coil when both heating and hot water are called for.
The tee would be either central heating return back to common return or simply a bypass although I would say it is the former.