A classification scheme is like a controlled vocabulary. In METEOR, classification schemes are a component of value domains. They establish consistent terminology, so everyone is using the same language. Classification schemes allow complex topics to be recorded accurately in value domains.

Many diseases have multiple names. For instance, motor neurone disease is also called Lou Gehrig's disease and amyotrophic lateral sclerosis. For a value domain to be useful, the terminology used for recording instances of this disease needs to be consistent.

Highly sophistacted classification systems such as the International Statistical Classification of Diseases and Related Health Problems Tenth Revision (ICD-10) are regularly maintained and understood by everyone internationally. To record the data in a data element, you need to use the classification scheme linked to the data element's value domain.

For example, have a look at the data element Emergency department stay—principal diagnosis, Emergency Department (ICD-10-AM Eleventh Edition) Principal Diagnosis Short List code ANN{.N[N]}. In an emergency department, a patient's principal diagnosis needs to be recorded. Instead of writing the diagnosis out in words – maybe it was a heart attack, which is also called a myocardial infarction – medical information management staff allocate the ICD-10 code for the particular diagnosis, using the value domain that will align with it. In this instance, the value domain is Emergency Department (ICD-10-AM Eleventh Edition) Principal Diagnosis Short List code ANN{.N[N]}. By using these codes, which are linked and consistent across Australia (and internationally, in the case of ICD-10), we can find out how many people in Australia suffered a heart attack in the past year, for instance.