Sully..Ditra can be installed both ways......but it performs better installed above the UFH....
quote from schluter..
Good to know Dave - I've only used it to overcome issues with new screeds (fast track, whereby the screed hasn't had the full cure time) and in that case, it sits below the UFH - according to Schluter tech. It needs to be flooded with slc as the cables shouldn't come in contact with it. Out of interest, in what spec would you sit it above the UFH? I also assume the cables would need to be flooded with slc as again, the ditra shouldn't come in contact with them as far as I'm aware.
As for using it to overcome floorboard movement, if that's what you are planning Bathroomboy - that's not what it's designed to do. I personally would overboard with a tilebacker board / marmox insullation boards, fix mats, flood with SLC and tile (although some guys are happy to fix directly over ufh mats with adhesive - I can't see the benefit or justify the risks of damaging the cables).