The fieldbus application layer (FAL) provides user programs with a means to access the fieldbus communication environment. In this respect, the FAL can be viewed as a “window between corresponding application programs.” This International Standard provides common elements for basic time-critical and non-timecritical messaging communications between application programs in an automation environment and material specific to Type 20 fieldbus. The term “time-critical” is used to represent the presence of a time-window, within which one or more specified actions are required to be completed with some defined level of certainty. Failure to complete specified actions within the time window risks failure of the applications requesting the actions, with attendant risk to equipment, plant and possibly human life. This International Standard defines in an abstract way the externally visible service provided by the Type 20 fieldbus Application Layer in terms of a) an abstract model for defining application resources (objects) capable of being manipulated by users via the use of the FAL service, b) the primitive actions and events of the service; c) the parameters associated with each primitive action and event, and the form which they take; and d) the interrelationship between these actions and events, and their valid sequences. The purpose of this International Standard is to define the services provided to the FAL user at the boundary between the user and the Application Layer of the Fieldbus Reference Model. This International Standard specifies the structure and services of the IEC fieldbus Application Layer, in conformance with the OSI Basic Reference Model (ISO/IEC 7498-1) and the OSI Application Layer Structure (ISO/IEC 9545). Although these services specify, from the perspective of applications, how request and responses are issued and delivered, they do not include a specification of what the requesting and responding applications are to do with them. That is, the behavioral aspects of the applications are not specified; only a definition of what requests and responses they can send/receive is specified. This permits greater flexibility to the FAL users in standardizing such object behavior. In addition to these services, some supporting services are also defined in this International Standard to provide access to the FAL to control certain aspects of its operation.