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!
No idea what the button is and I found nothing about it in the Hardware Maintenance Manual
https://download.lenovo.com/pccbbs/mobiles_pdf/t480_hmm_en.pdf
The system board has the part id 01YU859
https://pcsupport.lenovo.com/se/sv/parts-lookup?partId=01YU859&subsource=sharelink
Scribd has the schematic of the board, but I’m not technical enough to use it and answer your question.
https://www.scribd.com/document/619356079/Lenovo-ThinkPad-T480-01YR328-NM-B501-rev0-1
Noice! I was in fact looking for that information too! :D