Advanced user guides are "problem-oriented" docs in which users learn how to answer questions or solve problems. The major difference between these and the new user guides is that these guides are geared toward more experienced or advanced users who have more technical needs from their documentation. These users already have an understanding of Rancher and its functions. They know what they need to accomplish; they just need additional guidance to complete some more complex task they they have encountered while working.
It should be noted that neither new user guides nor advanced user guides provide detailed explanations or discussions (these kinds of docs belong elsewhere). How-to guides focus on the action of guiding users through repeatable, effective steps to learn new skills, master some task, or overcome some problem.