Thanks nux. My previous post wasn't one of my better efforts

I saw a helpful summary recently that captures the situation well.

For a particular fare to apply to the desired itinerary, two things need to coincide:

1. the fare rule must be adhered to, e.g. min/max stay, adv purchase, change & cancellation restrictions, permitted flights/routings/carriers, point of sale, valid dates of travel, combinations, etc, etc,

and,

2. there must be availability in the appropriate selling class (aka fare bucket) at the buyer's point of sale