iPhone 5s leftover part after battery replacement

I’m used to having leftover parts after an auto repair, but not so much for iPhones. I followed the steps to replace the battery on my wife’s 5s, and everything went smoothly until I was about to reattach the battery connector when I noticed a leftover piece at the top left of the chassis. I didn’t remove it, and I can’t find in an exploded parts diagram. I put the phone back together, and so far it seems to work fine. Can someone educate me about this part and the necessity of putting it back? If so, where does it go?