U+0353 Combining X Below

The Wikipedia has the following information about this codepoint:

The Uralic Phonetic Alphabet (UPA) or Finno-Ugric transcription system is a phonetic transcription or notational system used predominantly for the transcription and reconstruction of Uralic languages. It was first published in 1901 by Eemil Nestor Setälä, a Finnish linguist.

UPA differs from the International Phonetic Alphabet (IPA) notation in several ways.

The basic UPA characters are based on the Finnish alphabet where possible, with extensions taken from Cyrillic and Greek orthographies. Small-capital letters and some novel diacritics are also used.

Unlike the IPA, which is usually transcribed with upright characters, the UPA is usually transcribed with italic characters. Although many of its characters are also used in standard Latin, Greek, Cyrillic orthographies or the IPA, and are found in the corresponding Unicode blocks, many are not. These have been encoded in the Phonetic Extensions and Phonetic Extensions Supplement blocks. Font support for these extended characters is very rare; Code2000 and Fixedsys Excelsior are two fonts that do support them. A professional font containing them is Andron Mega; it supports UPA characters in Regular and Italics.