completion-styles is a variable defined in `minibuffer.el'.
Its value is
(basic partial-completion emacs22)

List of completion styles to use.
The available styles are listed in `completion-styles-alist'.

Note that `completion-category-overrides' may override these
styles for specific categories, such as files, buffers, etc.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.1 of Emacs.