Differences between restricted repositories and partner repositories?

Solution 1:

The restricted archive may contain only hardware drivers, not application software. It comes as a compromise in recognition that potential Ubuntu users could not use Ubuntu at all without it. Source: Ubuntu Policy Manual section 2.2.2.

The Canonical partner repository is not part of Ubuntu. It contains anything that Canonical thinks might be useful to Ubuntu users that is not Free Software. This includes application software.

So: restricted is a compromise in Ubuntu to make hardware work, and is restricted to just this. Canonical's partner repository is not part of Ubuntu, but may contain a wider range of non-free software.