question about UVN camera system

books say: N = targetPosition-camPosition let V'' = (0,1,0) U = V'' cross N V = N cross U do we arbitrarily or intentionally choose (0,1,0) as V'' ?is it just to get an U that''s perpendicular to N?then there''re a whole plane of Us(and hence Vs) that are perpendicular to N.why do we have to choose THE U made this way?