الهامش is the margin of the page, the space around the text that is left blank.
الحاشية is the text that is written in the margins. Historically it was written in the top, bottom, and outer margins. Today it’s mostly in the bottom.
And just if you are interested, المتن is the block of text/pictures in the middle of the page.
التذييل, historically meant endnotes, not footnotes (notes are all around the متن, as mentioned earlier), however, some people today might use it for footnotes. I highly advise against that as it could be confusing since many still use it as endnotes.
الملاحظة is any note, including any notes within the متن itself.
As for register, I’d say “educated”, as you wouldn’t even need to learn these words if you can’t read! Still, since the vast majority of people today know how to read, I’d have to say that anyone who actually reads more than magazines, road signs, and a novel or two per year should know most if not all these terms. Although I must admit that some still might confuse footnotes with endnotes, and endnotes with glossaries (فهرس) and glossaries with lists of contents (محتويات).
Nonetheless, الحاشية is not only the correct term, but also the least likely to be confused.
