eXo Platform uses PicoContainer, which implements the Inversion of Control (IoC) design pattern. All eXo containers inherit from a PicoContainer. There are mainly two eXo containers used, each of them can provide one or several services. Each container service is delivered in a JAR file. This JAR file may contain a default configuration. The use of default configurations is recommended and most services provide it.
When a Pico Container searches for services and its configurations, each configurable service may be reconfigured to override default values or set additional parameters. If the service is configured in two or more places the configuration override mechanism will be used.
If you are still confused, see the Service configuration for beginners section to understand the basics.