Document ID's are useful when you want to associate a metadata property to a document, or document set, which can then be used to retrieve the item regardless of where it is moved within the site collection, or even moved to a document center or record center.

The first step necessary to configure this functionality is to activate the Document ID site collection feature. You then run the Web Application specific timer jobs (Document ID assignment and Document ID enable/disable jobs) to assign a Document ID to every new Document and Document Set created. If the document or Document Set already existed they will be assigned a Document ID the next time they are updated.

Note: After activating the Document ID feature you should modify the default prefix, (because no-one would be able to remember this randomly generated prefix) for the generated Document ID's. This is accomplished by accessing the Document ID Settings option within the Site Collection Administration category of Site Settings. This prefix must contain 4-12 letters and numbers.

However, when you attempt to activate the Document ID site collection feature you may receive an Access Denied error.

This error is generated if the Create Subsites permission has been removed on the web application that contains the site collection that you are trying to activate the Document ID feature on.

To resolve this error you need to allow the Create Subsites permission for the web application. This can be done via Central Administration->Manage Web Applications->Web Application name ->User Permission. Within the Site Permission category check the Allow checkbox for the Create SubSites permission.

You will now be successful in activating the Document ID site collection feature on all site collections within the Web Application.