is simply the application of multiple policies within a specific section of the configuration + protocol default policy
Policy Subroutines
JUNOS software policy language is similar to a programming language
this similarity also includes the concept of nesting your policies into a policy subroutine
router first evaluates the subroutine and then finishes its processing of the main policy
evaluation of the subroutine simply returns a true or false Boolean result to the main policy
because you are referencing the subroutine as a match criterion, a true result means that the main policy has a match and can perform any configured actions
false result from the subroutine, however, means that the main policy does not have a match
actions in the subroutine do not actually accept or reject a specific route. They are only translated into a true or a false result. Actions that modify a route’s attribute, however, are applied to the route regardless of the outcome of the subroutine.