Vertical Left Square Bracket

It is the vertical form of the opening square bracket [.

Japanese, Traditional Chinese (especially in Taiwan and Hong Kong) and, historically, Korean are often typeset vertically: characters run from top to bottom, lines from right to left. In that layout a bracket has to open downward over the text instead of to the right, so the bracket is rotated a quarter turn. ﹇ is that rotated shape stored as a character of its own; its closing partner is ﹈. The same family holds vertical parentheses ︵ ︶, curly brackets ︷ ︸, double angle brackets ︽ ︾ and the vertical corner brackets ﹁ ﹂ that stand in for quotation marks.

Dropped into a horizontal line, as on most of the web, it no longer reads as a bracket at all. It looks like an overline with a tick at each end: a frame or lid, not punctuation.

Should I use ﹇ or just [?

Use [ (or the fullwidth ［ in Chinese and Japanese); the layout handles it.

Software that supports vertical writing, such as many word processors, e-book readers and browsers, takes the ordinary bracket and displays a vertical glyph for it, so you type the normal bracket and let the layout turn it. ﹇ is a compatibility character: its block exists mainly so text from older East Asian encodings, which stored vertical forms as separate characters, can be converted to Unicode and back without loss.

Because it is its own character, ﹇ can break things that work with [: a plain search for [ skips it, simple spellcheckers and code parsers may not treat it as a bracket, and if no installed font has it you see a blank box. Text clean-up tools that apply compatibility normalization (NFKC) fold it back to [, so a cleaned-up copy may silently lose the shape.