If your sources.list identifies the 'onofficial' repo... as "testing", ...But what if package X is the same in testing and unstable but there is a newer version in the 'onofficial' repo...
how do I install that package?
deb http://www.unofficial_repo.com testing main
...it will install automatically
A typical example might be cinelerra
Your preferred release is "testing". Your sources.list might look like this:
deb http://ftp.us.debian.org/debian/ testing main contrib non-free
deb http://ftp.us.debian.org/debian/ unstable main contrib non-free
deb http://ftp.cinelerra_repo.org/ unstable main contrib non-free
# aptitude install -t unstable cinelerra
...would evaluate all the pieces of cinelerra from all three sources, and take the pieces from "testing" that matched the highest version, then the pieces from Debian "unstable" that matched the highest version, then the rest of the pieces from the cinelerra repo since its versions were the very highest.
That's slightly oversimplified, but not much.