As i came across this code,
so wanted to know why and when to use this kind of key.
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This is a Japanese modifier key which will change between Hankaku (half-width) and Zenkaku (full-width) characters. To try and visualise this:
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As to which key it is bound to on the keyboard, F9 and F10 are common bindings. F10 will change from Zenkaku to Hankaku, and F9 from Hankaku to Zenkaku.
Source - Wikipedia