My HTC has three holes on the back cover, two at the bottom near where the Google logo is and a third just to the outside if the rear camera. Beneath are electrical contacts. What are they?

I would upload a photo, but this forum won't allow me. Some security error on the phone app. So, I will attach a link to a phone review that mentions them. The reviewer wasn't sure what they were either.

About a third of the way down there are some photos of the back with the cover on and off. Any ideas anyone?