This milestone defines 2nd stage of AE development.
During 1st stage (which lasted from the beginning) we were working on most base functionality of the software - mostly not including any game related features.
During this stage we're going to focus on the base of game related features' engines, such us: items, skills, actions, npc AI and some small additional features.
After this stage we'll have all mentioned systems working. But it doesn't mean that all skills will be working - it'll mean that the base system for skills is working. Having skills working will mean to define for example all effect handlers (what will be an easy work [from the coding point of view], but as there'll be a lot of effect handlers [I guess so], they we'll be being done for a long time ). The same is for AI - we'll have the AI engine working, what doesn't mean we'll have ai for all npcs/monsters. These will have to be created ( and again - it'll be easy work (from the coding point of view) [just creating some event handlers], but will require game related knowledge ( how which npc/monster behave etc ), so it'll be being created for long time, when new info is being gathered )