Wherever I have worked, MDD always stands for maximum dry density. With SP in front, not to be confused with USCS, there is little doubt in my mind that SPMDD is Std Proctor maximum dry density. I guess MPMDD is Modified Proctor maximum dry density. Now if you work in UK or India, etc, they use LT or HT for light or heavy tamping rather than Std Proctor or modified proctor . . .