”下付き” is a word which is not commonly used in daily conversations. It has two basic usages. +++Usage1: "Subscript" Please look here. https://support.content.office.net/ja-jp/media/29c7588e-57ed-4ef5-be8b-0908071f9e96.jpg The ”2” in ”H2O” is written in 下付き文字, and　”Ⓡ”　in "上付き" +++Usage2: That's the usage I should refrain from explaining here. For details, google it with "下付き "

I'm not a native speaker, so not sure, but this is what the dictionary gives: 下付き文字 (したつきもじ) - subscript So, basically, I guess it means something along the lines of "attached below". Hope this helps.