The location /jcr:system is reserved for use as a “system folder”. Some implementations may use this location for storing or exposing repository-internal data in content. For example, if a repository exposes node type definitions in content, then those node type definitions should be located at /jcr:system/jcr:nodeTypes.
If a repository supports versioning, then it must expose the version storage at /jcr:system/jcr:versionStorage (see 8.2.2.1 jcr:versionStorage).
If /jcr:system is supported its node type is left up to the implementation.