This is a specific design principle in computer science. It implements that the mechanisms should be separate from the policies, so as to not dictate them. Most commonly, this is incorporated in security management, but can also be used in different problems. Originally, Per Brinch Hansen introduced the system, which decouples the mechanism implementations. This enables different policies to use the same mechanism.
Instead of replacing the entire system, they simply replace the module, which dictates the policy. Hence, two-level implementation can be conveniently used to separate mechanism from policy. In addition, where there are different policies at play, it makes sense to have an understandable means of labeling policies, instead of simply identifying them by their code.