Policies are a powerful fine-grain mechanism that allow you to customise the behaviour of this library according to your needs. There is more information available in the policy tutorial and the policy reference.
Generally speaking unless you find that the default policy behaviour when encountering 'bad' argument values does not meet your needs, you should not need to worry about policies.
Policies are a compile-time mechanism that allow you to change error-handling or calculation precision either program wide, or at the call site.
Although the policy mechanism itself is rather complicated, in practice it is easy to use, and very flexible.
Using policies you can control:
You can control policies: