A server instance is a Application Server running in a single Java Virtual Machine (JVM) process. The Application Server is certified with Java 2 Standard Edition (J2SE) 5.0 and 1.4. The recommended J2SE distribution is included with the Application Server installation.
It is usually sufficient to create a single server instance on a machine, since the Application Server and accompanying JVM are both designed to scale to multiple processors. However, it can be beneficial to create multiple instances on one machine for application isolation and rolling upgrades. In some cases, a large server with multiple instances can be used in more than adminsitrative domain. The administration tools makes it easy to create, delete and manage server instances across multiple machines.