EF generally gets married segment inventory right as long as you search by origin and destination. As noted above, you need to beware with domestic FC fares as there is dual-inventory checking (both FC and coach buckets). Looking at FC buckets alone is not enough. You need to look at the FC fare filings and the underlying coach fare classes (first letter of fare basis code) and there must be inventory in both the FC booking class and underlying coach fare bucket for you to be able to book the FC fare. Z fares generally have U/T/X/V fare basis codes (in order from most expensive Z fares to cheapest), and so there must be inventory in both Z and U/T/X/V coach buckets to book them. I fares generally have Q/K/L fare basis codes and require inventory in those coach buckets. For completeness, D generally have M/H, C have B, and J fares use Y.