What is mean of timing model in EDA Gate Level Simulation?

--- Quote Start ---

When I use ModelSim-Altera to do simulation, I run modelsim from Quartus II using Tool-> Run EDA Simulation Tool -> EDA Gate Level Simulation. Then there are three options：

Slow-81.2V 85 Model

Slow-81.2V 0 Model

Fast-M 1.2V 0 Model

I just wonder what is difference between them and how can I decide which one I should use?

--- Quote End ---

All three.

Your systems should be designed to be robust over all process, voltage, and temperature variation (also known as PVT).

http://en.wikipedia.org/wiki/process_corners

These three timing models are different PVT setups.

Cheers,

Dave