From the introduction to the book:
Milieu 0 details the central persons, groups, and events of the opening years of the Third Imperium. Within these pages, you will find a wealth of information to bring this backdrop to life in your own campaign.
The infancy of the Third Imperium, when it consists of a mere handful of member worlds, together emerging from the shadow of the Long Night, jointly believing in their destiny to reestablish civilization throughout known space, to match legacies of Imperoiums pas, and to reach far beyond them. It is a time rife with opportunity for the bold and adventurous, when individuals can influence the fates of worlds.