The Libraries module displays library projects, which do not have a selector specified. These projects absorb the selector of any project which calls them; they are typically called by other projects via inline chains or pass/fail chains.
From the Libraries module, you can view, edit, create, or launch library projects. You can execute a library project by itself, but you must specify a selector when you do so.
You can change a library project into a normal project by editing the project and choosing a selector for it. When you save a library project with a selector, it becomes an ordinary project, disappearing from the Libraries list.
Aside from the lack of a selector, libraries are treated just like any other project.