Service Policy is a way to provide quality of service (QoS) in WebSphere. For example, let's say you classify your apps into different tiers.
In the left panel of the WebSphere admin console, select Operational policies > Service policies. The Default Service Policy cannot be deleted, nor can the importance of the Default Service Policy be changed, so it makes sense for the Default Service Policy to be used for your Platinum mission critical apps.
In this example, it would make sense to create a Gold service policy and a Silver service policy. Then, platinum apps would map to the default service policy, gold apps to the gold service policy, and silver apps to the silver service policy. When apps in the default service policy are using most of the available memory or CPU, requests to the apps apps in the Gold and Silver service policies would either not process or will process with lower priority.