A is deep discount FC or the most restricted instant UP fare, below P class. Most A fares are really something like H-UP. I is discount/deep discount business class, more expensive than the deep discount special S that isn't available in all markets. I think both S and I book into A for domestic connections. The I means that the flight is marketed as business because it's the continuation (after an aircraft change) of an international long haul flight, so that people booking both segments with BE for the long haul can get a through I fare. It's nevertheless domestic FC service.