Owner Identity

Hi, I have two query:
  1. What is exactly owner identity means?
  2. How to add Identity/User to Owner Identity? because When I run "Create New Revision" Action on "Released" state of a Document Item(logged-in as a user who created the document), got this Error  "You must be a member of the Owner identity to perform this action." Is there any difference between the owner of the Item and Created By Item.
  Regards, Maddy.
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