Board Thread:Destiny Forum/@comment-8456133-20150117051323/@comment-24057866-20150119201917

The information about the other expansions is speculation based on unofficial information and is therefore highly unreliable. Bungie explained the third subclass slot: "some people may recall that we used to have a mechanism for sealing a build and locking your choices in, which would give your build a boost. The game would let you make another copy of that build. That's what the slot is there for." A third subclass was just a wild guess that someone came up with before this explanation.