dash is portable in the sense that Debian needs it to be: they can port it to the architectures and systems that they need it to work on.
dash is also portable in the sense that you need it to be: it implements the POSIX shell spec and not much more, so it's likely that you have access to all its features when you need to use another shell as well.
dash is also portable in the sense that you need it to be: it implements the POSIX shell spec and not much more, so it's likely that you have access to all its features when you need to use another shell as well.