That is a very good summary. There are a few caveats - even within seemingly unrestricted classes - because there are subclasses, and the fare basis usually but doesn't always start with the booking code. Examples of "irregularities" may be when there's a promo for a full Y fare with an automatic upgrade, or when the fare is day of week restricted (like a C1234 fare to asia). It is always excellent to get the entire full rules for your fare - the only exceptions are F/FUA; C/CUA; Y/YUA. These are just about like cash. The rest will have some sort of other rule.