In my reading of "the booking code alone (the letter - N in your case) no longer defines the product. Cabin, fare brand, and fare class are separate constructs." seems to infer that there is no relation between fare buck and cabin. My experience would indicate that there is a correlation between fare bucket and cabin.

Now, how this relates to upgrade priority or fare brands is something I will not touch. In addition, there is usually a correlation between certain fares buckets and flexibility, J & Y for example, but I also won't get into that.

And yes, I did not include fare buckets (for example R, I, X, et al.) but also did not indicate my list was complete. I thought we'd all get the gist.