I find some of the alliances in the new edition of Warhammer confusing.
Looking at Tau, for instance, they are Battle Brothers with Eldar and Spacemarines, although neither of these are, to my knowledge allies of the Tau in the fluff.
Likewise, I noticed that Eldar are Battle Brothers to Dark Eldar...hang on, I though that these were deadly enemies of one another?
Also we have the blood angels - necron pairing (fluff), and the Sisters of Battle - Grey Knights as allies of convienince only (anti-fluff).
Are these pairings in the rulebook based on fluff? On the mechanics of each army? Something else?