If you are thinking of the id for the primary keys then core data handles this for you. On each object there is an objectID property you can access to see it. When you create an object from the managed object context, core data assigns a temporary id. When you commit the changes from the managed object context, core data assigns a perminant id. I don’t think it would be a good idea to manually try and set this.
Read this stackoverflow thread for more details.