U+0324 Combining Diaeresis Below

The Wikipedia has the following information about this codepoint:

The diaeresis ( dy-ERR-ə-sis, -⁠EER-; also known as the trema) and the umlaut () are two different diacritical marks that (in modern usage) look alike. They both consist of two dots ¨ placed over a letter, usually a vowel; when that letter is an i or a j, the diacritic replaces the tittle: ï. In computer systems, both forms have the same code point (binary code). Their appearance in print or on screen may vary between typefaces but rarely within the same typeface.

The "diaeresis" and the "umlaut" are diacritics marking two distinct phonological phenomena.

• The "diaeresis" diacritic is used to mark the separation of two distinct vowels in adjacent syllables when an instance of diaeresis (or hiatus) occurs, so as to distinguish from a digraph or diphthong.
• The "umlaut" diacritic, in contrast, indicates a sound shift phenomenon – also known as umlaut – in which a back vowel becomes a front vowel.

Neither of these phenomena occur in English, except in loanwords (like naïve) or for stylistic reasons (as in the Brontë family or Mötley Crüe).

These two diacritics have different origins, the diaeresis being considerably older. Nevertheless, in modern computer systems using Unicode, the umlaut and diaeresis diacritics are encoded identically. For example, U+00E4 ä LATIN SMALL LETTER A WITH DIAERESIS represents both a-umlaut and a-diaeresis.

The same mark, placed above or below the letter, is used in other contexts and for different purposes and meanings. For example, in Albanian, ë represents a schwa.