1. A system for policy-based service management, comprising: a hardware device including a rules definition interface module configured to receive a plurality of rule definitions stored in a data storage, wherein a rule definition includes a condition containing a rule definition variable; a policy management interface module configured to allow a user to define a rule instance from a rule definition and to define a policy instance based on one or more defined rule instances, wherein the user defines the rule instance by associating the condition in the rule definition containing the rule definition variable with one or more values in a user-defined value set; and a policy decision interface module configured to evaluate the policy instance upon request, for access to a network or a network resource by a subscriber device, wherein the policy decision interface is configured to retrieve one or more rule instances associated with the policy instance and to substitute one or more values in the user-defined value set associated with each variable in the rule instance in real-time when the rule instance is being evaluated by the policy decision interface module.