3副本和R(6,3)对比，为何3副本是LN+RR?而没有LR?

I can not understand it.For example,if 2-missing,the value of column named 3-replication  is 1LR+1RR.Why not 1LN+1RR?What it means?

Remarks:LN=localnode,LR=localrack,RR=remoterack