From the back of the book (2nd edition):
At last, extensive rules for the personnel and starships of the interstellar navies of the universe. Detailed character generation for naval characters, including the academy, and medical school, and 5 new skills. Rules for starship design, construction, and combat, with tonnages ranging to one million tons!
High Guard, created for the interstellar naval adventurer and referee.
Be sure to look for Book 4, Mercenary, with its coverage of the army, the marines, and of military adventures.
This book covers Imperial naval characters and ships of the Imperial navy in the Traveller setting.