Each code letter (booking class; fare bucket) can have its own set of rules (like requiring a saturday night stay-over). But the sets of rules could be almost the same for some of the code letters.
For any given trip, each letter has one fare associated with it (not including add on fees like for unaccompanied minors or seat selection) although the fare could change overnight or even in the middle of the day.
There can be different fares for a given letter for the trip A to B, the trip A to C connecting to C to B, and of course the trip from A to B only, and the trip from B to C only.
Sometimes a code letter comes and goes depending on when special fares are offered. For example U is Delta's everyday low price while T is offered or not offered or offered from time to time for fare sales.
Usually the code letters are ranked (not alphabetically) such that the allotment of seats under a code letter is actually the sum of the allotments of that letter and the letters below it. That is, if you need a particular level to receive a particular privilege such as no Saturday night stay, and if the seats supposedly belonging to that level are gone, a seat will be taken from a lower level allotment if available (you still get your chosen letter). If all the seats at your chosen level and below are gone, then you must choose (or the on line system will assign and charge you for) a higher level letter.
