Firstly, the caveat I always give: those figures aren't the numbers of unsold seats, they're what BA is prepared to sell. They may wish to oversell (expecting no-shows, etc) or be prepared to roll people forward (op-ups).

The letters themselves are effectively sub-divisions of each cabin for the purposes of variable pricing. For the WT+ cabin, the ones you are interested in are W, E and T (in descending order of price). For example, if a cabin is selling well, they may close out the lower buckets to capture more revenue from the less price-sensitive people. Or the reverse if demand is slack.

And the figure 9 ? It means "at least 9", so you'll never see a greater figure.

You may also see 'fare bucket' referred to as 'selling class' on ba.com.

Make sense ?