Bailey Unicorn unknown part

Can anyone tell me what this item is for. I eventually got round to reading my instruction book after six years of ownership because I have a slow puncture in one of the tyres and I couldn't get the wheel off. I phoned Bailey and they told me the caravan comes with locking wheel nuts for which the key was inside the blue bag with all of the instructions. Inside the bag I did indeed find the wheel nut key along with this green thing. By the way I have owned various caravans for over thirty years and have suffered two previous punctures but  simply called out the breakdown service (of which I am a member) who willingly changed the wheel, none of which had locking wheel nuts.

It seems I know nowt about photographs and iPhones either. The photos won't download because they have to be less than 500kb whatever that is. How do I know what kb they are and how do I make them that size or is that another discussion?

The green thing is a round plastic cap with the words 13P parking, Closed and Open with directional arrows.