Apparently Delta is using public inventory buckets J Z P G (and non-public bucket OY in some cases) to determine GUC/RUC availability. However once they have confirmed you the system converts the segment in your reservation to a different bucket. Here is the class mapping:
GUC Usage
MC or C+ purchased / Upgrade to DPS / P class needed / Books into RV (Clears at T-24.15 into D1 as OX if OY available)
MC or C+ purchased / Upgrade to D1 (no DPS) / Z class needed / Books into OK
DPS purchased / Upgrade to D1 / Z class needed / Books into OK
MC or C+ purchased / Upgrade to FC (no D1/DPS) / J class needed / Books into OV
RUC Usage
MC or C+ purchased / Upgrade to DPS / G class needed / Books into RX (Clears at T-24.15 into D1 as OX if OY available)
MC or C+ purchased / Upgrade to D1 (no DPS) / OY class needed / Books into OX
DPS purchased / Upgrade to D1 / OY class needed / Books into OX
MC or C+ purchased / Upgrade to FC (no D1/DPS) / Z class needed / Books into OL