The whole concept dates back to the days when there were only two fare buckets, F and Y, plus a few APEX Y fares. SEMN fares were booked in Y at a 40% discount, fully flexible and with a 40 kg luggage allowance.

Nowadays, travel agents who specialize in booking seafarers negotiate their own SEMN fares with the airlines, so the 40%-off concept no longer applies. The fares depend on the bucket they are booked into, but retain the flexibility and (I think) extra luggage allowance.

I rarely travel on SEMN tickets, in the long run it is cheaper to book regular inflexible tickets and chuck the odd one if plans change. I have a lot of colleagues who fly SEMN longhaul, though.

Johan