For ours they summarized how they know us and how long, as well as what social or family things we all do together. Also what they know of our future plans together when we return to Canada.
Some of ours were notarized and some were not. I think we had about 6 notarized letters and 4 that weren't.
(What country specific guide are you talking about? Did i miss something? The Document Checklist mentions letters as one form of proof of relationship...)
my rep said it would be nice if your common law spouse write a nortarized letter to talk about you and the relationship plus my letters the better it is
My partner and I also each wrote a letter as well as all the ones from our friends and family.
If you go here:
http://www.cic.gc.ca/english/information/applications/guides/5289ETOC.asp
and scroll to 'Step 2: Gather documents' there will be a link for "country specific requirements" and that should hopefully be what Bcboundboy was referring too.
I did mine before the new system came in and it looked a little different but I think that's the same thing.