What is the meaning of ꞉?

Alternative form of · for vowel length (e.g. ⟨a꞉⟩).

A component of the vowel letters ⟨a꞉⟩ and ⟨o꞉⟩ in a number of languages Papua New Guinea (e.g. Gobasi ⟨Go꞉bosi⟩).

The grammatical tone marking past tense.

A letter of the Cayuga alphabet, written in the Latin script, indicating vowel length; found in the letters a꞉ e꞉ ę꞉ i꞉ o꞉ ǫ꞉.

A letter of the Halkomelem alphabet, written in the Latin script, indicating vowel length and gemination in consonants.

the palatal click, modern ⟨ǂ⟩ (Besides the single letter, ⟨꞉⟩ is also used in digraphs for the palatal click consonants ⟨꞉k ꞉g ꞉n ꞉h ꞉kh ꞉ch⟩.)

A letter of the Kiowa alphabet, written in the Latin script, indicating vowel length.

A letter of the Kuni-Boazi alphabet, written in the Latin script, indicating vowel length.

Low lexical tone, on a syllable ending in a vowel or in any consonant but glottal stop.

The grammatical tone that marks a subject.

A letter of the Seneca alphabet, written in the Latin script, indicating vowel length; found in the letters a꞉ ä꞉ e꞉ ë꞉ i꞉ o꞉ ö꞉.

A letter of the Yele alphabet, written in the Latin script, marking nasal vowels.

Source: wiktionary.org