Once a contact is assigned to a Contact Owner it will stay assigned until it gets re-assigned (or un-assigned).  

A result code with "No" assignment will leave a contact un-changed.  If you want a result code to put a contact back into the general pool you would make the user assignment go to "No User".

A result code with a "Schedule a Call Back" Result Action BUT "No" assignment will not assign a contact to a Contact Owner.  It will only schedule that particular calendared event to the user who scheduled the call back.  This field is called the "Scheduled Call Back User" and is cleared once the call back in completed.