The arcanum is where the stories build. There are different types of story, journal entry type stories and story arc stories. The main difference is that the journal entry stories are built primarily by the Arcanum with little architect input and direction other than how the game is played. The story arcs are created as a collaboration between the architect and the arcanum.
The story arcs have their own timeline. Whilst not meant to be read sequentially the timeline gives the sequencing so they can be.