Edit: How could I miss that this was your first post?! I beat our very special corporate-wage-slave to greeting you too. Welcome to Flyertalk. Make yourself at home and please do contribute. In 2014, FT saved me a fortune, so the knowledge on here is literally invaluable!

You're pretty much spot on with your analysis.

When you get into the realm of flexible tickets, the difference between them is the degree of flexibility, and as you quite rightly point out, the availability of seats in the particular fare bucket.

The fare basis code actually determines the ticket conditions and price. For mere mortals (and most FT'ers on discount I class tickets) the fare basis codes are the regular published ones, with rules that FT'ers have come to automatically know (e.g. ex-UK I class fares have zero flexibility).

However there is nothing stopping you negotiating a special fare with BA that books into I class and is completely flexible (changes/cancellations anytime with no penalty). Obviously calling them up for 1 ticket won't yield this, but for TA's and corp clients it becomes interesting.

Your observation about First being cheaper than regular fares in higher CW buckets is correct. In fact for FT'ers booking ex-EU fares, the regular A class fare is often not much difference in price to the flexible R class fare.

There are a few things which differ between fare rules:
- Cancellations
- Changes
- No Show Rules
- Min/max stay requirements
- Advance purchase requirements
plus whatever else I can't quite remember right now.

Hope this helps