thermostat ratings , what do they mean?

When you purchase thermostat for the benz, how do you determine which one to pick up when you are given a choice of different ones. By
different i mean that temperature rating. For example choice1. 192Degree-Fail Safe choice2. 167Degree-Fail Safe choice3. 184Degree-Fail Safe
What do these temperature ratings refer to. How do you make a wise choice. Thanks.
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