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I don’t know why it’s called ruby. If you know Japanese you will know this as “furigana”.

It’s mainly used for added readings to words written using kanji.

"Ruby" is the name of the size of the type

To expand on that for those that don't know, traditionally all font sizes had names, like Great Primer for 18 pt, Ruby for 5.5 pt, and many others. https://en.wikipedia.org/wiki/Traditional_point-size_names