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-time-critical messaging communications between application programs in an automation environment and material specific to Type 25 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 document defines in an abstract way the externally visible behavior provided by the 
different Types of the fieldbus Application Layer in terms of: 
a) the abstract syntax defining the application layer protocol data units conveyed between communicating application entities, 
b) the transfer syntax defining the application layer protocol data units conveyed between 
communicating application entities,  
c) the application context state machine defining the application service behavior visible 
between communicating application entities; and 
d) the application relationship state machines defining the communication behavior visible 
between communicating application entities.dition 1.0 2019-06 INTERNATIONAL STANDARD colour inside Industrial communication networks – Fieldbus specifications – Part 6-25: Application layer protocol specification – Type 25 elements IEC 61158-6-25:2019-06(en) THIS PUBLICATION IS COPYRIGHT PROTECTED Copyright ？ 2019 IEC, Geneva, Switzerland All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or util