Originally Posted by flytoeat
In the second example, my understanding is that there are only 3 seats as S2 is a subset of Y3.
That's absolutely correct. The fare classes are nested. For Y and below, every fare class is a subset of the fare classes to the left of it. That's why 0's generally appear from right to left. If someone offers to buy an expensive fare (even when cheaper ones are available), you don't want to turn them down!
Of course, there are always a handful of exceptions to this rule.