While checking under the hood, I checked this screw and found it was fairly loose so I tightened it by hand. Does anyone know what this screw's function is? How tight are you supposed to keep this and is this an overflow vent?

Thanks in advance!

 

It looks like the same tank as the S Type, so it'll be the bleed screw.

 

It's the same tank. Also used on the XF. Confirm that you keep this screw tight unless you are bleeding the radiator?

Thank you!

 

Yes, that's correct keep it tight.