Originally Posted by Brobbel

Just an update: on Sabre I saw also the I-class.

I contacted FB for an explanation which classes were used.

For an award ticket on KL they use W for economy and D for business (as stated)
For an upgrade ticket on KL you have to book B or S, for availability they look at Z or D.
For an open miles ticket on KL they look to the C (business) or S (economy) class.