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

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).

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 (string, integer or decimal). 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 2: Abstract model of a compositional grammar expression

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 3: The main parts of an example expression