> 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/design/4-logical-model.md).

# Logical Model

A SNOMED CT Compositional Grammar expression begins with an optional definition status, contains one or more focus concepts (represented by a concept identifier) and optionally has a refinement. Each refinement may contain grouped or ungrouped attributes (or both). An attribute[^1] consists of the attribute name (represented by a concept identifier) together with the value of the attribute. The attribute value is either an expression or a concrete value (i.e. string, integer, decimal or boolean). Figure 2 below illustrates the overall structure of a compositional grammar expression using an abstract representation. Please note that no specific semantics should be attributed to each arrow in this abstract diagram.

<figure><img src="/files/6yu78vw5M4Jfx4Eufqky" alt=""><figcaption><p><strong>Figure 2: Abstract model of a compositional grammar expression</strong></p></figcaption></figure>

Figure 3 below shows an example of an expression with the main components marked. These components will be explained further in the subsequent sections of this document.

<figure><img src="/files/vDI19uUI9FSf5cDRXmQG" alt=""><figcaption><p><strong>Figure 3: The main parts of an example expression</strong></p></figcaption></figure>

## Details

Figure 4 below provides a more formal representation of the logical model of SNOMED CT Compositional Grammar using a UML class diagram.

<figure><img src="/files/KqJeCtieCLZWJ6KMWXQR" alt=""><figcaption><p><strong>Figure 4: Logical Model of SNOMED CT Compositional Grammar</strong></p></figcaption></figure>

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[^1]: Please note that, in the context of an expression, the word 'Attribute' is used to refer to the name/value pair within a refinement. However, in the context of the SNOMED CT concept model, the word 'Attribute' is often used to refer specifically to the 'attribute name' (i.e. the concept used as the relationship type).


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