What it means is that NW will keep trying to sell those seats right up to til the last minute.

Once the EUA (upgrade) bucket is empty then upgrades happen at the gate in priority order.

If NW releases additional seats into the EUA bucket then the nightly computer run will start with plats first, then golds, then silvers.

Plats who purchase tickets late in the game (say less than 3 days) before will often ride in coach since the gold EUA run will have already filled most of first.

NW's EUA system as currently built favors those who can plan ahead (and pay the least) and amazingly (to me at least) punishes last minute flyers who pay the most for their fares. The new 'R' bucket is designed to reduce this problem... but it cannot get rid of it entirely.