The ElyrianMUD will be for everyone that wants to participate. It was intended to help provide some of the story background for the game when it is released.
The Kingdoms of Elyria will be reserved basically for the Nobility and Aristocracy, a Dance of Dynasties. This will allow the leaders of Elyria to make treaties, packs, alliances, of course enemies as well. This should drive the conflicts in the social structure of the game when it is released.
The Prologue will be a downloadable single person experience that takes place in one small area of the game, soft of a playable demo so you can get a handle on the UI and other game mechanics. the Prologue should be available between the Alpha and Beta phases when ever that will be.
And the Exposition will be a 3 month early access for certain tiers (mainly Nobility and Aristocracy) to help set up the world in preparation for launch. This is where the physical structures are layed out and upgraded, some resources are utilized (river driven lumber mill for example) and possibly the introduction of roads, bridges and other structures. So basically the players during the Exposition will be working hard on the Infrastructure of the game so when the players join in on day one, the game should be ready to go.
Hope this helps with the understanding of the different pieces of the game and it is set today. But please be advised that all the above information can be changed at any times by the Devs to stay tuned.