Actually, since some categories (such as @review and @suppbook from biblatex) do not necessarily have titles of their own, it may be best to include a few other fields in the sort sequence to accommodate them. The following sequence may be better:

```
publications.sortmethods.authortitle = {
    sequence = {
        { field = "author",    default = "",           unknown = "" },
        { field = "title",     default = "",           unknown = "" },
        { field = "booktitle", default = "",           unknown = "" }, -- if this is an untitled section (e.g., introduction, foreword, preface) of a book or a review of a book
        { field = "maintitle", default = "",           unknown = "" }, -- if this is an untitled section or volume in a multivolume collection
        { field = "volume",    default = "",           unknown = "" },
        { field = "part",      default = "",           unknown = "" },
        { field = "date",      default = "9998-13-32", unknown = "9999-14-33" }, -- some specifications allow date instead of year, month, day
        { field = "year",      default = "9998",       unknown = "9999" },
        { field = "month",     default = "13",         unknown = "14" },
        { field = "day",       default = "32",         unknown = "33" },
        { field = "index",     default = "",           unknown = "" },
    },
}
```

It is also worth noting that in order for the volume field to sort properly, its values will have to be padded with leading zeroes; otherwise, an entry with volume = {2} will be sorted after one with volume = {10}. To accommodate this, specifications would need to remove the leading zeroes when they typeset volume and part numbers.