Purpose of black PCGS sticker

I have a coin that was slabbed by PCGS in 2010. The attached image shows the label on the back side of the slab.
Just curious. What is the purpose of the black PCGS sticker on the right side of the label? It is attached on the outside of the slab, but I do not know if it was attached at the time the coin was slabbed, or sometime later.
If the sticker is a security feature, what does it do? It’s not an NFC chip.
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Comments
It's a hologram sticker attached by PCGS at the time of grading.
Thanks, but what’s its purpose?
It's an authentication device, also might be reactive to uv light.
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Did a little research, and found that PCGS introduced hologram stickers in 2014. If interested, here is their announcement.
And here is a short video.
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