Just read this on the Canon Professional Network tips page and wanted to share as I had never heard of it before.
"If you plan on using a battery grip with your EOS DSLR camera, then you'll need to remove the battery door cover on the camera body so the grip can fit. The problem is that if you lose that little door, the camera will not work when you later want to shoot without the battery grip. To avoid losing the battery door cover, the battery grips feature a small slot. Simply slide the battery door cover in there and it will be kept safe with the camera and grip, ready for use should you decide to remove the battery grip to shoot without it. "
The holder is along the side of the part of the grip that goes into the battery compartment of the camera (marked by an arrow pointing towards the body of the grip). The non curved end of the battery cover slides into the body of the grip and click into place.
"If you plan on using a battery grip with your EOS DSLR camera, then you'll need to remove the battery door cover on the camera body so the grip can fit. The problem is that if you lose that little door, the camera will not work when you later want to shoot without the battery grip. To avoid losing the battery door cover, the battery grips feature a small slot. Simply slide the battery door cover in there and it will be kept safe with the camera and grip, ready for use should you decide to remove the battery grip to shoot without it. "
The holder is along the side of the part of the grip that goes into the battery compartment of the camera (marked by an arrow pointing towards the body of the grip). The non curved end of the battery cover slides into the body of the grip and click into place.