C class fares are like D class on other carriers. I am not sure about AC, but on BA discount business would apply if you upgrade a holiday package etc. So for example a BA D class fare is not a "published" fare in that you can not walk into an agent and buy the ticket. But it is available if you book your trip as BA Holidays (automatically priced at Y) and upgrade to business (D) or first (A) for a surcharge. I would imagine AC uses C as an omnibus fare class for upgrades, packages, some companion tickets and so on. Sorry I dont have too much detail - I hope this helps