This character is a Other Letter and is mainly used in the Bopomofo script.
The glyph is not a composition. Its East Asian Width is wide. In bidirectional text it is written from left to right. When changing direction it is not mirrored. U+31B3 offers a line break opportunity at its position, except in some numeric contexts.
The Wikipedia has the following information about this codepoint:
Bopomofo (ㄅㄆㄇㄈ), also called Zhuyin (Chinese: 注音; pinyin: zhùyīn), occasionally Mandarin Phonetic Symbols (Chinese: 注音符號; pinyin: zhùyīn fúhào; Wade–Giles: chu4yin1 fu2hao4), is a Chinese transliteration and writing system for Mandarin Chinese and other related languages and dialects. More commonly used in Taiwanese Mandarin, it may also be used to transcribe other varieties of Chinese, particularly other varieties of Mandarin Chinese dialects, as well as languages like Taiwanese Hokkien. Consisting of 37 characters and five tone marks, it transcribes all possible sounds in Mandarin.
Bopomofo was first introduced in China by the Beiyang government in the 1910s and was used alongside the Wade–Giles system for romanization purposes, which used a modified Latin alphabet. Today, Bopomofo is now more common in Taiwan than on the mainland, and is after Hanyu Pinyin used as a secondary electronic input method for writing Mandarin Chinese in Taiwan as well as in dictionaries or other non-official documents.