Parking light bulb- whats the cap thing for?

[sasolshares]

Hi,
I have Xenon headlamps and one of my parking light bulbs went, The bulb is behind a rectangular removable cover and is a 3 prong 30mm diam bayonet fit with a red O ring seal. The replacement consists of the bulb but with a separate black cap, also bayonet fit with a red O ring but slightly larger diameter than the bulb (about 40mm diam).
Anyone know what this extra bit is for? Can't see anything like it on the headlamp assembly. I would have asked the dealer but someone else picked it up for me.

thanks
Rob
Facelift E53 3.0D