What is this button?

Edit: this guide doesn’t seem to mention it https://pcsupport.lenovo.com/us/en/products/laptops-and-netbooks/thinkpad-t-series-laptops/thinkpad-t480-type-20l5-20l6/20l6/20l6s01q2y/document-userguide

I was flashing Libreboot on to my ThinkPad T480, when I noticed this little button. Sorry for the poor images!