bibtex-sort-entry-class is a variable defined in `bibtex.el'.
Its value is
(("String") (catch-all) ("Book" "Proceedings"))

  • This variable is safe as a file local variable if its value
    satisfies the predicate which is a byte-compiled expression.

List of classes of BibTeX entry types, used for sorting entries.
If value of `bibtex-maintain-sorted-entries' is `entry-class'
entries are ordered according to the classes they belong to. Each
class contains a list of entry types. An entry `catch-all' applies
to all entries not explicitly mentioned.

You can customize this variable.