DIF fares all appear something like TEUX35R/DIF4 in ExpertFlyer, eg here for LHR-MCT WT+ fares with dual inventory required in WT:
Code:
Fare Basis Airline Booking Class Trip Type Fare Cabin Effective Date Expiration Date Min / Max Stay Adv Purchase Req
TLXZ90S7F/DIF4 BA T Round-Trip 158.00(GBP) PE 01/02/2018 30/11/2018 -- / 12M 7
NNXC00S7 BA N Round-Trip 166.00(GBP) E -- / 12M
TLWZ90S7F/DIF4 BA T Round-Trip 188.00(GBP) PE 01/02/2018 30/11/2018 -- / 12M 7
NNWC00S7 BA N Round-Trip 196.00(GBP) E -- / 12M
SNXC00S7 BA S Round-Trip 216.00(GBP) E -- / 12M
TNXC00S7F/DIF4 BA T Round-Trip 226.00(GBP) PE -- / 12M 7
But if you are using ITA Matrix only the part before the / is shown when it tells you which fare covers each part of the journey. However I have yet to see a DIF fare with the prefix part before the / that was shared with another fare, so you can still work out which fare it is.
BA's website or callcenter or airport staff are the only places to buy these fares and there they simply show up as very cheap WT+ fares.
Expertflyer is bad at showing the fare rules for these fares, for example it will not show fare rules for most of the WT+ DIF fares, nor will it show the booking classes required.