How do you calculate Pairwise XOR?

Question

How is pairwise XOR calculated ? I googled but could not find anything relevant.

EDIT : How is it different from normal XOR?

Answer 1

Each bit of the number are pairwise XOR'd with each other. For example:

  15 = 0000 1111
  21 = 0001 0101 

So, the XOR is

       0001 1010 = 26