It is not necessary to use all the components in the set. Users can decide which components in the set they would like to use. Interfaces to other applications can be managed in the following manner:
Applications can obtain information / use the services of the component by using the public APIs provided
Applications can respond to events exposed by the component
Components come with default implementations of methods that call on the services of assumed external systems. Applications can extend/change these default implementations to implement interfaces to any external system


The steps involved in using a desired set of components are:
Customize the Installation bean to set all the components desired to be used to ‘Yes’
Use the components in your application

For more details on the usage of components and interfacing with other applications, refer Installation and Usage document