First rule: it doesn't show what's 'available', but what the airline is prepared to sell ... which may be two completely different things, depending on your definition of the term 'available'.

A better definition might be "available to buy" rather than "available to sit on" (btw, 9 means "at least 9")

However, you can infer/guess/assume (!) that the front of the aircraft is pretty busy (F = F,A, CW = J,C,D,R and I).

WT+ (W, E and T) is fairly open.

Lower Y fare buckets (N,O,Q and S) are closed out because historical data, etc tells BA they can sell the seats at higher fares.

Make sense ?

PS what a strange order for the display ...

Edited to add:

Where you see "W9 E9 T9", this doesn't mean 27 or more seats; it means 9 or more in the cabin as a whole, which BA are prepared to sell at any of the 3 fare buckets.