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Use the force (google) : Non Linear Delay Model, Synopsys Composite Current Source Model Comparing NLDM And CCS delay models - Paripath - improving sign-off
CCS mostly used for sign off analysis. It is precise, but too heavyweight library. Not all EDA tools even support it. And not all Fabs offer it for their processes.
The choice would depend on your process technology.
While the older nodes did just fine with NLDM (90nm+), the newer and smaller process nodes prefer CCS for better accuracy.