The document cache indicator is hidden by default. You need to enable it by checking "Enable the Document Cache Indicator" in → , General|Features section.
As of v4.9, XXE makes a heavy use of its document cache. This indicator allows to have a feedback while the document cache is caching documents. Its tool tip shows the number of documents currently cached as well as the capacity of the cache. The number of cached documents is also rendered visually.
This indicator is not a button. You'll have to use Advanced, "Cached data" section to configure or clear the document cache.→ ,