Quoting Dan Phillips (dwp@rushpost.com):
http://www.debian.org/releases/stable/i386/apds06.html.en
Can I make a suggestion that or maybe a change request for use of space key
or Enter key rather than + or - keys for expanding the options as referred
to in the above document. I am not the first person to be fooled by this
and most likely not the last. I do not believe using + and - is an
intuitive way of using these menus and not everyone will read the Appendix
Some people would laugh seeing me taking this example, but "+" is the
common way to deploy a sublist in hierarchical menus in many graphical
environments, including MS Windows (just try with en Explorer window
on any Windows flavour).