How are disk and memory caches used?
A. Depending upon the cache restrictions on the cache item, it will be added either to the disk or to the memory cache. If an item is marked as secure, and “dont cache SSL objects on disk” is set on or its a https: URL, then it will be stored in memory. All mail/news inlines are stored in memory. Only exception to this rule is when the object is set as a StreamAsFile for a plugin.