I am 90% sure that when Emirates give us a special economy class (a fare with a short selling life) that its booked in Q. As for Virgin it might be a premium economy special fare class. Every airline uses slightly different class's
for different cabins.
Example W on BA
is World traveler Plus Discounted Fare.
W on South African is the class they use to book Business when the client is paying for the upgrade with Voyager points. K class on BA
is economy were as
K class on Virgin is one of the class's they use for premium economy.
First Class - F, A
Business Class - C, J, D, I, Z,
Premium Economy - Virgin K
British Airways T, W,
EVA Air K, R,
(With upgraded economy class's there do not seem to be any set rule every airline is different)
Ecomony - Y, S, V, L, M, K, Q, B,
Then you have staff fares or industry fares, which again change airline to airline.
These are all economy
Qantas - N
British Airways N (used to be X)
Virgin Atlantic N
Iberia X (they use the same class for passengers buying a ticket with
frequent flyer points)