The Routing Policy represents a set of rules which defines the relation between the router and the external world in term of route exchanges and protocol interaction. To summarize, the choice of a routing policy allows the user to define:
The Routing Policy is an very important part of the gated configuration file, and should not be neglected especially in case of the use of several different protocols on the same router ( IGP, EGP, static routes, direct routes, etc...). Routing Policy is often referred as Route Filtering since defining a policy is equivalent to defining a set of filters to which to apply a route before accepting it or redistributing it.
The Imports policy defines the set of routes which is accepted. Criteria for accepting a route includes parameters like interface from which the route is received, address of the peer sending the route, autonomous system of the peer, etc... Section 4.26 explains the detail of the import statement for defining the incoming route filter.
The Exports policy defines the set of routes which is exported to the external world by any protocol. The criteria are the same as for the Imports policy.
The export policy also defines the interaction between the different protocols ran on the router. By default a given protocol will redistribute only the routes originated from that protocol. For instance RIP will announce only route learned from RIP, and BGP will announce only routes learned from the other BGP peers. The export policy will allow the protocols to exchange routes between each other, making external routes available for redistribution by internal protocols, and vice versa. The export policy is also very useful for advertising static routes (like a default route) through any protocol.
Section 4.27 explains the details of the export statement.
All the routing policy statements make use of Route Filter to explicitly define the set of routes accepted or announced. A Route Filter is merely a list of routes, in term of address (prefix) and mask, to which a given route will be matched. Route Filter are also used on the martians statements.