how is whence different than which?

I read https://unix.stackexchange.com/questions/9683/which-but-all/ but I cannot really get the difference. I am running zsh 5.4.2 on 64-bit debian-buster. Both which and whence are shell-builtins . Can people point out where whence would be more appropriate than which and vice-versa ? /home/shirish> zsh --version zsh 5.4.2 (x86_64-debian-linux-gnu) /home/shirish> type -a which which is a shell builtin which is /usr/bin/which which is /bin/which /home/shirish> type -a whence whence is a shell builtin