From the introduction:
The Jovian Chronicles Companion is a supplement for the science fiction game universe of the same name. As its name implies, the book is a companion volume to the main rulebook, offering additional insights and ideas on subjects that could not always be fully covered due to a lack of space. The solar system of the twenty-third century is vast and complex after all, and it is hard to provide much more than glimpses of it through these few pages.
The Companion contains advanced rules and specific cases for those who want to take their Jovian Chronicles characters and campaigns a step further. It also features additional background infomation, much of it visual in nature, and the long-awaited, complete Silhouette vehicle and weapon design systems. Design, not construction, because statistics are chosen and not decided by a set of formulas that may or may not match the concept you have in mind. There is no such restriction in these rules, allowing both Gamemasters and Players to field the vehicles they need for the story and not just close approximations. The design rules also handle only the "nuts and bolts" game part, leaving you free to describe your new creation in as much detail as you like.