The CacheTier class stores the cache server variables if using Memcache, or the cache filesystem variables if using Filecache, as well as a flag to determine if the connection is open, and the cache object returned from the connection function. By default there's a base and presentation tier, as well as any others that have been defined in the cache configuration. The tier objects are instantiated in the CacheFactory and stored in the MemcacheTiered or FilecacheTiered (or equivalent) object.