An unmanaged page means a page which have no control. That means a page without any admin or moderator. This types of pages can be claimed by a person.
If you want to use a page in facebook then you need to create a facebook page using your facebook profile. Or you can help other to manage a facebook page. For example you can act as moderator, editor and contributor to a page that is managed by other person. That means you are not an admin but you are posting on behalf of the page.
What is Unmanaged Page?
Some pages appears in facebook which don’t have any admin, moderator or editor. These pages are automatically created. For example you will find a page in facebook Named Wikipedia. There is no admin for this page. No admin is allowed for this page also. If anyone add a content to Wikipedia and want to show on facebook they can post to this page by mentioning or check-in.
Pages that are by name of a place, a book, a movie, a popular website and such others may be created automatically. But not always. These pages are created based on users request. For example you are the first person who is checking an extreme section of Himalayas. Then a page will be created with that name. You are reading a book which haven’t any page in facebook. But you want to update an status. Then a page will be created after your status. But if the owner created a page with same name people check-in or mention that one. So, in this case no page is created.
How to Claim an Unmanaged Page?
To claim an unmanaged page you should be the owner of that business. Because facebook verify this subject. If facebook found anything wrong you might be have a warning or block from facebook. You can only claim what is actually belongs to you. Such as a movie you have created or a book you have written.
If the page related to your business is unmanaged you will see an unofficial page tag below the page name. There you will find a option “Is this your business”. If this business is your, you can claim this by clicking this button and providing necessary information.