U+031D Combining Up Tack Below

The Wikipedia has the following information about this codepoint:

In phonetics and phonology, relative articulation is description of the manner and place of articulation of a speech sound relative to some reference point. Typically, the comparison is made with a default, unmarked articulation of the same phoneme in a neutral sound environment. For example, the English velar consonant /k/ is fronted before the vowel /iː/ (as in keep) compared to articulation of /k/ before other vowels (as in cool). This fronting is called palatalization.

The relative position of a sound may be described as advanced (fronted), retracted (backed), raised, lowered, centralized, or mid-centralized. The latter two terms are only used with vowels, and are marked in the International Phonetic Alphabet with diacritics over the vowel letter. The others are used with both consonants and vowels, and are marked with iconic diacritics under the letter. Another dimension of relative articulation that has IPA diacritics is the degree of roundedness, more rounded and less rounded.