CC now allows you to define a complex sort available in one tap on the sort menu. By "complex" we mean a sort that can specify up to 3 sub-sorts. For example, you might want to sort the book list so that books in a series are shown in series order while books not in a series are shown in author/title order. To do this you would set up a user defined sort with the following subsorts:
- Major (first): series. This will sort by series and series index.
- Second: author. Books with no series (or with identical series/series index) will be sorted by author.
- Third: title. Books with identical authors will be sorted by title.
You define a user-defined sort in CC's settings.
You can use a user-defined sort as an auto-sort ( What does "auto-sort" do when grouping?).