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A non-discoverable (or "private") item is one that is simply hidden from all search/browse/OAI results, and is therefore only accessible via direct link (or bookmark).  By default, all Items are discoverable, meaning they will appear in search/browse/OAI results.

It's important to clarify that non-discoverable items may or may not be access restricted.  It is possible for an Item to be publicly availableanonymously visible, but non-discoverable, so that you can only publicly access the item if you are given a link to it.

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