This would be the AmEx International Airline Program that operates through PTS/CTS for Plat/Centurion cardholders and also now has a website feature. AFAIK you do need to pay with an AmEx card of some sort and must have Plat/Centurion card to use the program. The CX premium economy tickets might have some sort of bulk or consolidator designator which inhibits upgrades. I know my fare codes through this program end in a slack and about four additional letters/numbers. The discounted fares for these are not considered to be published, which means that they cannot be upgraded with miles in at least some FF programs.

There's a thread on this (abbreviated IAP) in the AmEx forum.