[CentOS] what is extra/ updates/ and weak-updates/ under /lib/modules/<kernel>/ directories?

Hi,

 there are three 'extra' directories under
/lib/modules/<kernel>/ directories for each kernel
version. Any one can explain what is the real
functions for these three directories, and how to use
them correctly?

 I found a document talks a little about it at
http://linux.dell.com/files/presentations/Red_Hat_Summit_May_2006/Driver_Update_Model.pdf,
but would like to know more real alive examples to see
how I can really use the kernel driver update
mechanism.

 Thanks.



 
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