Mefieveet72
As there are several ‘half fare’ passes/travelcards, it’s just the term ‘half fare’ which matters for a ticket purchase, so the correct ½ price will be calculated.
Because you don’t need the half fare card yet for PURCHASING tickets. It’s not some ‘discount voucher’, whether you already have one is NOT checked at all during the purchase process. You need a valid half fare card only while TRAVELING with a purchased discounted ticket, so this would be only checked during a ticket control on board.