ISO/IEC 20933:2016 specifies:

1) an ID triggered modular access system, the functions of the modules and the messages they exchange, and the sequence of messages, i.e. transitions of the transaction;
2) the system responsibility from receiving an access request until sending the result. i.e. a complete transaction;
3) the responsibilities of the modules, including time stamping and responding to the requests they received; and
4) the sequence and semantics of the messages and their elements.

The electronic version of this International Standard can be downloaded from the ISO/IEC Information Technology Task Force (ITTF) website.