In GACS, concepts are labeled with natural-language phrases. Concept labels, or "terms", are preferred or alternative. Preferred terms can be used as tags for subject indexing (as "descriptors"). The precision of term-based indexing depends on the uniqueness of the strings used to encode the terms. In production environments, for example when thesauri are used in complex workflows, it is important that preferred terms also remain stable.

Terms other than preferred terms are non-preferred, or alternative ("non-descriptors"). Because non-descriptors are used to direct users to preferred terms, they are also known as "lead-in-terminology". In standard thesaurus practice, non-descriptors are not used to tag resources for indexing and retrieval.

In accordance with thesaurus practice, SKOS prescribes that concepts not have two preferred labels in a given language and recommends that preferred labels in a given language be unique within the context of a given concept scheme (e.g., that no two concepts share the prefLabel "lime"@en). In accordance with the SKOS specification, it is an error if a concept has a same literal both as its preferred label and as an alternative label.

In GACS:

  1. Preferred labels MUST be unique for the core languages of GACS. That is, one concept MUST NOT have the same preferred label as another concept in the same (core) language. Note: "unique" is case insensitive.

  2. Preferred labels SHOULD be unique for the other (non-core) languages of GACS. That is, one concept SHOULD NOT have the same preferred label as another concept in the same (non-core) language.

  3. In core languages, an alternate label MUST NOT be identical to a preferred label of another concept.

  4. In other (non-core) languages, an alternate label SHOULD NOT be identical to a preferred label of another concept.

  5. Alternate labels MUST be unique for the core languages of GACS. That is, one concept MUST NOT have the same alternate label as another concept in the same (core) language.

  6. Alternate labels SHOULD be unique for the other (non-core) languages of GACS. That is, one concept SHOULD NOT have the same alternate label as another concept in the same (non-core) language.