Output, formatting language and sorting

Here too, the trend is to offer the user hundreds of ready-made styles rather than a powerful and rich formatting language, and once againg a powerful language is one that can handle conditions ("IF something is absent THEN do that"). A basic language is always incorporated into these packages and citation styles can be modified or created from scratch but they tend to perform and assure the minimal performance. It is fashionable to state that the package can produce HTML and XML outputs: presently the quality, the complexity of such HTML or XML tagging can be truly deceiving.

"Subject bibliography" traditionally refers to a list which is not only sorted by one or more nested criteria, but a list where the sorting key (i.e. the criteria) is also a heading out of context. It is routine in large bibliographies and catalogs where the key used to sort the items is clearly displayed on top:

Here the heading (the "subject" of the bibliograpic list) is the author name: Adam, D.M. followed by a counter for the references where it is recorded "(2)".
Two nested levels of sorting headings would be a plus, normally lacking. If you can replace the full bibliographic references displayed under the headings by a reference, for example, to the record number,  you get an index: very useful.

Sorting records is essential to handle data, it comes just after searching. Sorting records via more than one nested criteria (first date, if date is the same then sort by authors, under the same author sort by main title etc.) is very important (and it is another example of 'hidden' "IF...THEN" clause).
Sort implies and can often hide other important factors regarding the way characters are handled: main heading lacking in records, length of the sort key, case, digits, leading articles and non-filing characters, letter's sequence according to the selected language (Spanish sorts different from Italian), letters with diacritics ...