Terminology experts usually maintain their own Code Systems. These can be independent or dependent on international or other terminologies, for example on SNOMED CT, and are called extensions. Mostly, extensions contain modules that are assigned to additional concepts which have their terms written in the maintainer’s native language, or terms for specific usages. Snow Owl supports maintaining multiple levels of dependent extensions and allows isolating them from each other. Moreover, multiple extensions on the same level can also exist simultaneously. For example, it is possible to maintain two SNOMED CT extensions that are based on different versions of the same international SNOMED CT version.