IEC 61158-5-21:2010 specifies the structure and services of the type 21 IEC fieldbus Application Layer, in conformance with the OSI Basic Reference Model (ISO/IEC 7498) and the OSI Application Layer Structure (ISO/IEC 9545). This standard cancels and replaces IEC/PAS 62573 published in 2008. This bilingual version published in 2012-01 corresponds to the English version published in 2010-08.