user agent layer (X.400) (UAL)
The User Agent Layer (UAL) is the application-dependent sublayer of the OSI reference model. It acts as an interface in MHS systems according to X.400 between the user interaction and the Message Transfer Layer( MTL). The International Telecommunication Union( ITU) recommends the Interpersonal MessagingSystem( IPMS) as the application. An instance of the User Agent Layer (UAL) is the User Agent Entity( UAE).