You can use collections to arrange publications into categories and sub-categories. You can also get detailed statistics about each collection such as publication timeline and important authors for a particular set of publications. A publication can be part of multiple collections because you may be reading a publication which is related to two different topics.
You can create collections by pressing the "+" button next to the "Collections" node on the left tree, or by using the right click menu. You can rename the collection by double clicking on them.
You can copy or move publications between the collections in your library. The simplest way is to drag and drop the publications into a collection or you can use the right click menu options.