I have about 10 different possible story threads that your group could assume. Your choices will determine which one you do. Once you are on a story thread the milestones in that thread will be set. Once you achieve all the milestones you qualify for a downtime break. Some story arcs will span through multiple other story arcs. But whenever you complete one you get this downtime break.
A downtime break allows for training. Specifically levels and possibly other things. You cannot train levels, even if you have the requisite XP unless you accomplish at least some form of story arc. We are going to see how this work out.
There will also be personal story arcs. This is intended to allow heavy XP earners to be able to go ahead and train their levels if they need to. Feel free to ask questions about this here. This is new territory for me as well and we are play testing it.