Manual:$wgCategoryCollation

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Used when the collation algorithm changes to determine which categorylinks rows need updating.

Currently supports uppercase (make everything uppercase for sorting), identity [in 1.18+] (sort by binary value of string when stored as utf-8. Essentially sort by code point), and uca-default (complex, much more multi-lingual friendly category collation) collations.

Since MediaWiki 1.18 extensions can add extra collations via the Collation::factory hook.