The application instance( AI), Application Entity (AE), is located in the application layer of the OSI reference model and represents the totality of user element, application service element of the OSI reference model and control functions for simple mappings. All aspects of an application process relevant for communication are modeled in the application instance.
An application instance is an active element within the implementation of the application layer. It consists of a number of application service elements( ASE) that collectively describe the communication functions, and a user element that supports those parts of the application process that use the application service elements.
A distinction is made between application-independent service elements, Common Application Service Elements( CASE), and application-specific service elements, Specific Application Service Elements ( SASE). CASE contain general service elements that can be assigned to many applications; SASE are called the service elements that are tailored to specific standardized applications.