How to create a context sensitive DFF segment on Discrete Job for 1 WIP Accting Class? eBS R12.2.10

Hi, I am trying to add a context sensitive DFF segment on Discrete Job which will only be enabled for a specific WIP Accting Class. We already have the DFF WIP_DISCRETE_JOBS configured without context (i.e. all dff segments available for all discrete jobs), but I want just 1 new segment mandatory based on the selection of a specific wip accounting class. Can anyone help with advice? I've tried following the Flexfield user guide but am getting nowhere.