Page "Collation" Paragraph 36

When letters of an alphabet are used for this purpose of enumeration, there are certain language-specific conventions as to which letters are used.

For example, the Russian letters Ъ and Ь ( which in writing are only used for modifying the preceding consonant ), and usually also Ы, Й and Ё, are usually omitted.

Also in many languages that use extended Latin script, the modified letters are often not used in enumeration.