Actually, I think monitor is correct on this point.

I asked some agents about the D fare code that we had been discussing around here at various points. It turns out that the D code is used for the YONEPASS fares (the "instant upgrade" full fares). Those have a higher priority for FC seats than the F codes, but lower than the A's. So, in essence, the order goes "priority," Platinum, Gold, Silver, "nobody" within each of the letter codes (A, D, F).

This does, incidentally, mean that a full-fare "nobody" can get the seat before a discounted Platinum.

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Michael