In GACS:

• Each concept in GACS is identified with a unique URI.
• Each concept has at least one preferred label, either tagged by language or designated as a scientific name.
• Each concept has only one preferred label per language. (Different language tags for a given language, such as en-us and en-gb, designate separate languages.)
• All labels in a given language -- preferred, alternative, or designated as scientific name -- are unique strings.
• Alternative labels are optional; hidden labels, though part of SKOS, are not used.