The Metathesaurus contains over one million concepts. Users create a useful subset, or smaller grouping of concepts, by choosing source vocabularies or applying a filter in MetamorphoSys, the free UMLS installation.

Examples of subsets include:

• Source vocabularies in a language (all Spanish vocabularies)
• All terms that are free for use within the United States
• CPT codes to be used for billing purposes
• Terms with the semantic type 'Clinical Drug'

There are two relational formats for Metathesaurus subsets:

• Rich Release Format (RRF)
• Original Release Format (ORF)

Developers are encouraged to use the RRF which better represents the detailed semantics of each source vocabulary and provides better representations of concept name, source, and hierarchical information.