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When a clinical finding is mentioned in a patient record certain assumptions are usually made about what it means in relation to the person who is the subject of that record. Thus if the finding 56018004 |wheezing| is present in a record it is assumed to mean that the subject of that record is wheezing at the time of examination. This assumed meaning might be stated in full "the subject of the record is currently wheezing" but a contracted form that omits explicit reference to the subject, timing and presence of the finding is more usual in written records.

Similarly when a procedure is mentioned in a patient record assumptions are usually made about what it means in relation to the subject of that record. Thus, in the absence of other information, the mention of the procedure 38102005 |cholecystectomy| may be assumed to mean that a "the subject of the record had a cholecystectomy at a stated time ".

Although default assumptions such as those above may be made, it is also possible for mention of the same finding or procedure to have a very different meaning. For example, "past medical history of wheezing", "not wheezing", "father suffers from wheezing", 183980003 |cholecystectomy planned|, |cholecystectomy not done|

The SNOMED CT context model provides a way to model concepts that explicitly state the clinical situation in which they are used. This same model also allows the construction of expressions that explicitly state the clinical situation in which a concept is being used in a particular record.

A proprietary record structure or a reference information model may also express aspects of context and these can be mapped to the SNOMED CT context model where appropriate to create comparable expressions

The context model also specifies a default context that applies to findings and procedures which are expressed in a patient record without any explicit statement of context.

The most important aspects of the context model are those which have the potential to express a meaning that differs fundamentally from the meaning associated with the default context. Changes to context that have this fundamental effect on meaning are referred to as "axis modifying". The phrase "axis modifying" indicates a change that shifts the meaning between different axes in the subtype hierarchy

The context model allows "axis modification" to be expressed within the general abstract logical model applied to all SNOMED CT concepts. To achieve this a concept such as 160303001 |FH: Diabetes mellitus| is modeled as a subtype of 281666001 |family history of disorder|. It is not a subtype of 73211009 |diabetes mellitus| but instead its association with the finding 73211009 |diabetes mellitus| is modeled using a defining relationship 246090004 |associated finding|. Similarly a 183992003 |Hip replacement planned| is a subtype of 405613005 |planned procedure| (not a subtype of "hip replacement"). It is related to "hip replacement" by an 363589002 |associated procedure| relationship

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