GlassFish Server creates one server instance, called server at the time of installation. You can delete the server instance and create a new instance with a different name.
For many users, one server instance meets their needs. However, depending upon your environment, you might want to create additional server instances. For example, in a development environment you can use different server instances to test different GlassFish Server configurations, or to compare and test different application deployments. Because you can easily add or delete a server instance, you can use them to create temporary “sandbox” areas to experiment with while developing.