It's just something I came up with to summarize two separate ways of attaining HAAs. Holy Pallies and Fire Mages have the Direct style because they loot the HAA directly off of some (particular) mob in a (particular) zone. Affliction Warlocks and Havoc Demon Hunters have the Indirect style because they only get the HAA indirectly after looting some other object off of some (particular) mob in a (particular) zone. In other words, these two specs get a specific item from that mob, and then proceed to use that item indirectly to get their HAA, rather than just looting the HAA directly off of the mob.
You'll note that much of the talk surrounding the Glowing Runestone sounds like the Indirect style, and so I originally thought that the Glowing Runestone was probably dropped by some (particular) mob in a (particular) zone, and would then indirectly lead us to Futhark and our HAA. I've lost confidence in this idea, though, since nobody has yet found either the Runestone or Furthark.