Best practice and style are at the core of every program; there is always a better way to do something. Policy helps programmers determine how something should be written.
Policy is a more or less ambiguous way of describing the best way to do something in programming. This can often be described as "best practice" when writing a program. This can pertain to the way a method returns a value, the way inheritance is structured, and the way to exit a loop, among other things, and it can vary from language to language.
You can learn more about policy and programming paradigm here.
