Y and U are different fare buckets in Economy. The actual difference is based on published fare rules, which you can see on ExpertFlyer or online if that fare is available. Very often, especially with EK, there is no difference between fares in terms of fees for changes, max/ min stay etc. Depends where you're flying out of. But higher fares can be waitlisted, for example, which is not possible on lower fares. You should be able to upgrade by paying fare difference from most fares, so that is accurate.

C is indeed revenue Business and entitles you to all the bells and whistles of Business travel.