The FAQ for [all](http://superuser.com/faq) [three](http://unix.stackexchange.com/faq) [sites](http://askubuntu.com/faq) seems to cover it pretty well.

Super User concerns itself with computer hardware, software, and personal home computing/networking; Unix and Linux concerns itself with *nix operating systems (not specifically a particular distro); and Ask Ubuntu concerns itself with questions in regards to Ubuntu and its official derivatives.

To answer your question more pointedly:

> [Ask Ubuntu] is not the right place for:
> 
>  - Linux Mint, Backtrack, and other Linux distributions (try our friends at Unix & Linux Stack Exchange).

...so you'd do well with Unix and Linux.