2.2.4 IDL Syntax Extensions
Extensions specified in sections 2.2.4.1 through 2.2.4.11 affect the syntax of the message, while extensions specified in sections 2.2.4.13 through 2.2.4.17 affect the processing of the message without directly changing the messages. Extensions specified in section 2.2.4.18 affects neither the syntax nor the processing of the message.