The applications form the third level of the J2EE Engine architecture. The boundary between the applications level and the J2EE Engine components level is defined by the J2EE 1.3 APIs along with a few SAP proprietary APIs. Applications use them to utilize the functions of the different types of components on the lower level in the system.
An enterprise application consists of several types of application components. These components reside in different containers on the J2EE Engine as shown in the J2EE Engine Containers figure below. Each of these containers provides runtime services and life cycle management for the application components.
SAP J2EE Engine Containers
Applications inform other applications or J2EE Engine components that they need to use their classes or runtime objects by defining references to them. Read more on the topic in Application References.