It looks like some information is still missing in fare screen:

For example, OW fares from NYC to MIA show a number of cheap Y-UP type fares which are not bookable (and this fares are shown as 'B' = business). Unfortunately the routing rules for US-domestic fares do not show applicable booking classes, which I think may answer the question in this case (the primary booking class [shown on main fare search screen] for Y-UP fares is usually amended by other letters in routing rules).

Any ideas if this is resolvable?