In Agrippa’s day, fencing as a martial skill was already in decline. He laments that the use of artillery precludes the normal means by which one earns military honor. So instead he focuses on the sword as a dueling weapon, for it is useful in both trial by combat and to defend oneself from unforeseen assault.
The three parts of fencing are Justice, Knowledge, and Practice. Aside from reminding the reader that the use of weapons is an “auxiliary” method of resolving problems and under the law should be used only as a last resort, he doesn’t intend to speak on that topic.
As for Knowledge and Practice, they appear to be the indented focus of his first and second book respectively. Flipping through the pages, I see that book one is indeed about principles and the images show a single fencer, nude, in ideal postures. The second book shows pairs of fencers in earnest combat. Some are nude, others fully attired.