The default is true to enable sticky sessions. Applications are self-contained subtrees of the web site. They have distinct Application objects, sessions, and servlet mappings. Applications can even be deployed entirely from .jar files. The full application configuration is described in a separate page.
Applications have a root app-dir similar to the document root.
By default, it's just the same as the relative path. The url prefix of the application. A regular expression matching a url prefix. The application's root directory. The url-regexp configuration allows for dynamic applications, for example, creating a new application for each user.