SharePoint 2010 - Document ID's

https://msdn.microsoft.com/en-us/library/ee559302(office.14).aspx

Document ID's in SharePoint Server 2010 provide you with the ability to tag documents with a unique identification number.

You enable this for the Site Collection by enabling the feature "Document ID Service".
Now you will see a new Site Collection Admin thing for Document ID Settings.
This means you can generate custom ID's. Very, very handy!

If the standard stuff is not enough, heck, you can now extend / write your own!
See: https://www.zimmergren.net/archive/2010/04/13/sp-2010-sharepoint-server-2010-creating-your-custom-document-id-provider.aspx