> For the complete documentation index, see [llms.txt](https://docs.snomed.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.snomed.org/snomed-ct-specifications/snomed-ct-compositional-grammar-specification/introduction/2-use-cases/2.2-expressions-in-messages.md).
# Expressions In Messages
The SNOMED CT Compositional Grammar allows SNOMED CT expressions to be represented as a text string that can be carried in messages. For example, in HL7 version 3 messages SNOMED CT expressions can be placed in the 'code' property of the ConceptDescriptor (CD) data type (Release 2). The compositional grammar described in this document is intended to replace the qualifier mechanism that formerly was in the CD data type (Release 1), and which was removed in Release 2.
In September 2009, the HL7 Version 3 Standard "Data Types - Abstract Specification, Release 2" was published for its fourth normative membership ballot. This revised standard defined what can be carried in the 'code' property of the CD data type as shown.
> **Definition of the 'code' property of the Concept Descriptor (CD) data type from HL7 Data Types Release 2**\
> **Code (code)** : ST.SIMPLE\
> **Definition:** The plain code symbol defined by the code system, or *an expression in a syntax defined by the code system which describes the concept***. *****(emphasis added)*** If provided, the code SHALL be an exact match to a plain code symbol or expression defined by the codeSystem. If the codeSystem defines a code or expression that includes whitespace, the code SHALL include the whitespace.
>
> ***An expression can only be used where the codeSystem either defines an expression*** ***syntax*****, or there is a generally accepted syntax for the codeSystem.** *(emphasis added)*
The syntax described in this document is intended to satisfy the need for a "syntax defined by the code system" as stated above, when the "code system" is SNOMED CT. For specific guidance on using SNOMED CT Compositional Grammar in the CD data type (Release 2), please refer to Appendix B.4 of "HL7 Version 3 Implementation Guide: TermInfo – Using SNOMED CT in CDA R2 Models, Release 1".
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