what is policy server

A policy server is a security component of a
policy-based network that provides
authorization services and facilitates tracking and control of files. The policy server accepts access control requests, processes them against a formal set of statements that define how the network's resources are to be allocated among its clients (known
as the policy), and returns access control responses.

In a policy-based network, the resources can be allocated by the policy server based on the time of day, client authorization privileges, availability of network resources, and any other factors that the network manager may specify when composing the policy.
With respect to files and documents, the policy server can allow or deny access, control the extent to which a client can use the document (for example, one-time printing or downloading only), track client use patterns, log changes or modifications, provide
automatic updates, eliminate unnecessary redundancy, minimize the need to re-issue documents, and delete outdated data. A policy server can be an important component of networks in which client privacy must be protected (as in medical records) or secrecy maintained
(as in military operations).

简单来说，就是权限控制服务器。