Ignition key fob

[Skateboard]

Sounds daft seeing as how I've now sold my car but the new owner is asking about the ignition key fob.
There is a 1.5" oblong black plastic "key" on the key ring which (to my embarrassment) can't remember what it's for ?? Is it an emergency key to disable the immobiliser ??
Anyone got any idea's please ??

[shake n bake]

Little socket below the ignition barrel, plug that plastic dooberry in and it disables the immobiliser for 20 seconds.

[Skateboard]

Ah right, I've never had to use it.
Thanks very much Shaky.