What is the glue overlay function?

[<sparklezou@hotmail.com>]

Dear all,

   I'm puzzled about how to use glue overlay. Is it used to support 
multiple naming contexts in the same DS?

   And in the Man, there is no example, so it's difficult to understand.

   Please give me some example. 

   Thank you very much!

   Waiting for your kind replay!

                                                     Best Regards,
                                                     Sparklezou