completion-category-overrides is a variable defined in `minibuffer.el'.
Its value is
((buffer (styles basic substring)))

List of `completion-styles' overrides for specific categories.
Each override has the shape (CATEGORY . ALIST) where ALIST is
an association list that can specify properties such as:
- `styles': the list of `completion-styles' to use for that category.
- `cycle': the `completion-cycle-threshold' to use for that category.
Categories are symbols such as `buffer' and `file', used when
completing buffer and file names, respectively.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.1 of Emacs.