It means the additive package in the oil is suitable for diesel engine use. CC, CD, CE, CF, etc. Oil rated for gasoline engines is rated SF, SG, SH, SJ, SL, SM, etc. The "C" stands for compression ignition while the "S" stands for spark ignition. Some oils meet both S and C requirements. Some newer "automotive" oils only meet the most current S requirements for gasoline engines.
Most current C rated oils also meet S requirements for older gasoline engines. In that regard, the "diesel" oil is better both for diesels and older gasoline engines. Don't use oil intended only for gasoline engines in a diesel.
Sorry this is not a real good explanation, but the way I understand it.
