Trouble with EKGs

I was wondering what the lower right lead is used for when running twelve lead EKGs.
1. Lead I is RA to LA
2. Lead 2 is RA to LL
3. Lead III is LA to LL
4. aVR is RA lead
5. aVL is LA lead
6. aVF is LL lead
and V1-6 are all unipolar leads which completes the 12 lead analysis

Then what is the lower right lead for anyway? I know it's used for something or else we wouldn't put it on obviously. Thanks for the help you guys.