I have to wonder why unstage isn’t defined as unstage = reset HEAD. That would save keystrokes and would mean you could just type git unstage FILE.txt. Is there a situation where you want to unstage to something other than HEAD?
unstage = reset HEAD
git unstage FILE.txt
HEAD is actually the default <tree-ish>/<commit> argument, so git reset FILE.txt and git reset HEAD FILE.txt should produce the same results.
git reset FILE.txt
git reset HEAD FILE.txt
To your question, there might be a situation in which one might want to unstage something other than HEAD, but I’ve never encountered it.