# Expressions With a Definition Status
A SNOMED CT postcoordinated expression may be used to express a clinical meaning that is either equivalent to or a subtype of the given expression.
## Equivalent To
When the combination of focus concepts and attribute refinements are both necessary and sufficient to define the clinical meaning being represented, the 'equivalent to' definition status is used. Expressions with a definition status of 'equivalent to' may be used to express a fully defined clinical meaning in a health record or a message, to represent the definition of a fully defined precoordinated concept, or to represent an association between LOINC and an equivalent SNOMED CT expression.
To indicate that an expression has a definition status of 'equivalent to', three consecutive equals signs (i.e. "===") are placed at the start of the expression. For example, the following expression represents the definition of the fully defined concept [31978002 | Fracture of tibia|](http://snomed.info/id/31978002) :
```
=== 46866001 |Fracture of lower limb| + 428881005 |Injury of tibia| :
116676008 |Associated morphology| = 72704001 |Fracture| ,
363698007 |Finding site| = 12611008 |Bone structure of tibia|
```
Because expressions are most often used to represent the full definition of a clinical meaning, 'equivalent to' is the assumed definition status when it is not explicitly stated. For this reason, the expressions above has an identical meaning to the following expression:
```
46866001 |Fracture of lower limb| + 428881005 |Injury of tibia| :
116676008 |Associated morphology| = 72704001 |Fracture| ,
363698007 |Finding site| = 12611008 |Bone structure of tibia|
```
Using 'equivalent to' as the assumed definition status helps to ensure the backward compatibility of this version of compositional grammar with the previous one.
## Subtype Of
When the combination of focus concepts and attribute refinements are necessary but not necessarily sufficient to define the clinical meaning being represented, the 'subtype of' definition status is used. Expressions with a definition status of 'subtype of' may be used to represent a primitive clinical meaning in a health record or a message, the definition of a primitive precoordinated concept, or an association between LOINC and a SNOMED CT expression that is necessary but not sufficient.
To indicate that an expression has a definition status of 'subtype of', three consecutive less than signs (i.e. "<<<") are placed at the start of the expression.
For example, the primitive precoordinated concept [46635009 | Type 1 diabetes mellitus|](http://snomed.info/id/46635009) can be defined using the expression:
{% code overflow="wrap" %}
```
<<< 73211009 |Diabetes mellitus| : 363698007 |Finding site| = 113331007 |Endocrine system|
```
{% endcode %}
The definition status of 'subtype of' on this expression indicates that the definition is not necessarily sufficient to define the concept [46635009 | Type 1 diabetes mellitus|](http://snomed.info/id/46635009) .
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