ACL, Difference between owner_sid and sid?

Hi
I am new to the ACL part of Spring security. I found this excellent blog post http://blog.denksoft.com/?page_id=20.
What I dont understand is the difference between the owner_sid in ACL_OBJECT_IDENTITY and sid ACL_ENTRY. I think I understand what the sid in ACL_ENTRY is used for and that is mapping a user or role with a CRUD operation to a specifik domain object. But what is the owner_sid for?

Thanks