Yes I noticed it too with QANTAS' "D" class for upgrades on its domestic sectors. I am a QANTAS FF member, and when I do upgrades with QANTAS using certs, I notice that the class they book me in is in "D".

CX seems to be only one who has gotten it right.. well sort of.

"A" and "D" class exists and CX uses them as revenue seats. Though "A" had the problem of being sold out faster than the regular "F" seats. But I think after my numerous complaints to CX yield control last year, CX has improved "A" and "D" class availability this year.

CX uses "Z" and "U" for First and Business award class. Mind you, they also use the same code for upgrades. It's funny how CX recognises what is upgrade and what is an award ticket. I've had points automatically credited on Asiamiles upgrades (say from J to F) correctly but yet, on another FF program upgrade, (say on AA's J to F upgrade award), I've had to send AA copies of ticket and explain that this is an upgrade as CX is adamant that it was an 'award' flight.