From the introduction:
"This volume presents twelve minor races for use in any Traveller Campaign. Each is suitable for non-player and player character status"
Each alien is described along with its homeworld, character generation rules, and encounters both with in The Imperium and on their native worlds.
While part of the Marc Miller's Traveller T4 core book series which uses the default setting of Milieu 0, most of the book is background material, rule system specific material is minimal, and none of the aliens home planets are specifically located rather are just given a general description and history. As such the material in this is easy to adapt to almost any Traveller setting, especially The Third Imperium.